staging: rtl8712: Change return values of r8712_setdatarate_cmd()

Change the return values of function r8712_setdatarate_cmd from _SUCCESS
and _FAIL to 0 and -ENOMEM respectively.
Change the return type of the function from u8 to int to reflect this.
Change the call site of the function to check for 0 instead of _SUCCESS.
Return the value at the call site directly instead of storing it in a
return variable.
Remove now-unused return variable.

Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>