MIPS: Fix rtlx build error.

  CC      arch/mips/kernel/rtlx.o
/home/ralf/src/linux/upstream-sfr/arch/mips/kernel/rtlx.c: In function ‘file_write’:
/home/ralf/src/linux/upstream-sfr/arch/mips/kernel/rtlx.c:439:23: error: unused variable ‘rt’ [-Werror=unused-variable]
/home/ralf/src/linux/upstream-sfr/arch/mips/kernel/rtlx.c: In function ‘rtlx_module_init’:
/home/ralf/src/linux/upstream-sfr/arch/mips/kernel/rtlx.c:523:3: error: implicit declaration of function ‘set_vi_handler’ [-Werror=implicit-function-declaration]
cc1: all warnings being treated as errors

Caused by 496ad9aa [new helper:
file_inode(file)].

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>