ath6kl: fix function name conflicts with ath9k

ath6kl: fix function name conflicts with ath9k

Stephen reported that compilation fails if both ath6kl and ath9k are
compiled in:

drivers/net/wireless/ath/ath6kl/built-in.o: In function `htc_start':
(.opd+0x600): multiple definition of `htc_start'
drivers/net/wireless/ath/ath9k/built-in.o:(.opd+0x3e40): first defined here
drivers/net/wireless/ath/ath6kl/built-in.o: In function `.htc_stop':
(.text+0x7b40): multiple definition of `.htc_stop'
drivers/net/wireless/ath/ath9k/built-in.o:(.text+0x67b34): first defined he=
re
drivers/net/wireless/ath/ath6kl/built-in.o: In function `.htc_start':
(.text+0x7d18): multiple definition of `.htc_start'
drivers/net/wireless/ath/ath9k/built-in.o:(.text+0x67ba0): first defined he=
re
drivers/net/wireless/ath/ath6kl/built-in.o: In function `htc_stop':
(.opd+0x5e8): multiple definition of `htc_stop'
drivers/net/wireless/ath/ath9k/built-in.o:(.opd+0x3e28): first defined here

To fix this add ath6kl prefix to all public functions in htc.c.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>