staging: mt7621-mmc: fix unused variable compiler warning

The compiler complains:

    drivers/staging/mt7621-mmc/dbg.c: In function ‘msdc_debug_proc_write’:
    drivers/staging/mt7621-mmc/dbg.c:237:12: warning: unused variable ‘size’ [-Wunused-variable]
      int mode, size;
                ^~~~
    drivers/staging/mt7621-mmc/dbg.c:237:6: warning: unused variable ‘mode’ [-Wunused-variable]
      int mode, size;
          ^~~~

Remove these declarations.

Signed-off-by: George Hilliard <thirtythreeforty@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>