staging: rtl8712: style fix returned error code

This patch fixes a coding style issues in the rtl8712 module as noted
by checkpatch.pl regarding the returned error code.

It fixes the following checkpatch.pl warning:

WARNING: ENOSYS means 'invalid syscall nr' and nothing else

Signed-off-by: Martin Homuth <martin@martinhomuth.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>