staging: rtl8712: make_wlanhdr(): Change return values and type

Change return values of make_wlanhdr from _SUCCESS/_FAIL to 0/-EINVAL.
Modify call site to check for non-zero return values instead of _FAIL.
Change return type from sint to int.

Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Link: https://lore.kernel.org/r/20190809052353.5308-4-nishkadg.linux@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>