staging: r8712u: Most return-values changed from -1 to proper errno macros.

The ioctl handlers were frequently returning -1 upon failure. Most of
these have now been changed to proper errno macros.
The few remaining ones have been left untouched because either the
handler is not called (and so cannot be tested), or the function never
fails (and so cannot be system-tested), or requires new code to
distinguish its failures.

Signed-off-by: Ali Bahar <ali@internetDog.org>
Cc: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>