rtlwifi: use s8 instead of char

rtlwifi: use s8 instead of char

Compiling the rtlwifi drivers for ARM with gcc -Wextra warns about lots of
incorrect code that results from 'char' being unsigned here, e.g.

realtek/rtlwifi/rc.c:113:18: error: comparison is always true due to limited range of data type [-Werror=type-limits]
realtek/rtlwifi/rtl8188ee/dm.c:1070:22: error: comparison is always false due to limited range of data type [-Werror=type-limits]
realtek/rtlwifi/rtl8192ce/trx.c:54:16: error: comparison is always false due to limited range of data type [-Werror=type-limits]
realtek/rtlwifi/rtl8192cu/mac.c:601:16: error: comparison is always false due to limited range of data type [-Werror=type-limits]
realtek/rtlwifi/rtl8192de/trx.c:53:16: error: comparison is always false due to limited range of data type [-Werror=type-limits]
realtek/rtlwifi/rtl8192ee/phy.c:1268:12: error: comparison is always true due to limited range of data type [-Werror=type-limits]
realtek/rtlwifi/rtl8192se/rf.c:150:20: error: comparison is always false due to limited range of data type [-Werror=type-limits]
realtek/rtlwifi/rtl8723be/dm.c:877:29: error: comparison is always false due to limited range of data type [-Werror=type-limits]
realtek/rtlwifi/rtl8723be/phy.c:386:16: error: comparison is always true due to limited range of data type [-Werror=type-limits]
realtek/rtlwifi/rtl8821ae/dm.c:1514:38: error: comparison is always false due to limited range of data type [-Werror=type-limits]
realtek/rtlwifi/rtl8821ae/phy.c:1558:11: error: comparison is always false due to limited range of data type [-Werror=type-limits]
realtek/rtlwifi/rtl8821ae/phy.c:386:24: error: comparison is always false due to limited range of data type [-Werror=type-limits]
realtek/rtlwifi/rtl8821ae/trx.c:55:12: error: comparison is always false due to limited range of data type [-Werror=type-limits]
realtek/rtlwifi/stats.c:31:16: error: comparison is always false due to limited range of data type [-Werror=type-limits]

This patch changes all uses of 'char' in this driver that refer to
8-bit integers to use 's8' instead, which is signed on all architectures.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>