staging: exfat: drop unused function parameter

sbi parameter not used inside the function so remove it.
Also cleanup unused variables generated by this change.

Signed-off-by: Valentin Vidic <vvidic@valentin-vidic.from.hr>
Link: https://lore.kernel.org/r/20190908173539.26963-1-vvidic@valentin-vidic.from.hr


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>