ssb: fix boolreturn.cocci warning

The coccinelle report
./include/linux/ssb/ssb_driver_gige.h:98:8-9:
WARNING: return of 0/1 in function
'ssb_gige_must_flush_posted_writes' with return type bool

Return statements in functions returning bool should use true/false
instead of 1/0.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Yang Guang <yang.guang5@zte.com.cn>
Signed-off-by: David Yang <davidcomponentone@gmail.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/fa4f1fa737e715eb62a85229ac5f12bae21145cf.1642065490.git.davidcomponentone@gmail.com