staging: rtl8188eu: return an error code, not a boolean

staging: rtl8188eu: return an error code, not a boolean

"If the name of a function is an action or an imperative command,
the function should return an error-code integer." (Documentation/CodingStyle)

Several action-like functions in this driver return a boolean: _SUCCESS = 1 on
success, _FAIL = 0 on error, defined in
drivers/staging/rtl8188eu/include/osdep_service.h.

Change rtw_start_drv_threads() to return a proper 0-or-error value.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Greg Kroah-Hartman <gregkh@linux.com>
Cc: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>