misc: sgi-xp: xp_main: Staticify local functions xp_init() and xp_exit()

misc: sgi-xp: xp_main: Staticify local functions xp_init() and xp_exit()

These functions are not exported and no external prototypes exist

Fixes W=1 kernel build warning(s):

 drivers/misc/sgi-xp/xp_main.c:227:1: warning: no previous prototype for ‘xp_init’ [-Wmissing-prototypes]
 227 | xp_init(void)
 | ^~~~~~~
 drivers/misc/sgi-xp/xp_main.c:250:1: warning: no previous prototype for ‘xp_exit’ [-Wmissing-prototypes]
 250 | xp_exit(void)
 | ^~~~~~~

Cc: Cliff Whickman <cpw@sgi.com>
Cc: Robin Holt <robinmholt@gmail.com>
Cc: Dean Nelson <dcn@sgi.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20200701083118.45744-5-lee.jones@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>