kgdb: Return true in kgdb_nmi_poll_knock()

Fix the following coccicheck warning:

include/linux/kgdb.h:301:54-55: WARNING: return of 0/1 in function
'kgdb_nmi_poll_knock' with return type bool

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Link: https://lore.kernel.org/r/20200507110649.37426-1-yanaijie@huawei.com


Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>