staging: r8188eu: remove unneeded variable

Eliminate the following coccicheck warning:
./drivers/staging/r8188eu/os_dep/os_intfs.c:505:6-12:
 Unneeded variable "status". Return "_SUCCESS" on line 577
./drivers/staging/r8188eu/os_dep/os_intfs.c:772: 4-7:
  Unneeded variable "ret". Return "_SUCCESS" on line 818
./drivers/staging/r8188eu/os_dep/os_intfs.c:823:4-8:
Unneeded variable "ret8". Return "_SUCCESS" on line 849

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Jing Yangyang <jing.yangyang@zte.com.cn>
Link: https://lore.kernel.org/r/20210825061531.69678-1-deng.changcheng@zte.com.cn
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>