efivars: Mark local function as static

This fixes the following sparse warning
drivers/firmware/efi/efivars.c:567:6: warning: symbol 'efivars_sysfs_exit' was not declared. Should it be static?

Signed-off-by: Bojan Prtvar <prtvar.b@gmail.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>