ssb: fix boolreturn.cocci warning

./include/linux/ssb/ssb_driver_extif.h:200:8-9:WARNING: return of 0/1 in
function 'ssb_extif_available' with return type bool

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

Generated by: scripts/coccinelle/misc/boolreturn.cocci

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Jing Yangyang <jing.yangyang@zte.com.cn>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210824061341.59255-1-deng.changcheng@zte.com.cn