efi_loader: fix incorrect use of EFI_EXIT()

efi_get_variable_common() does not use EFI_ENTRY(). So we should not use
EFI_EXIT() either.

Fixes: 767f6eeb ("efi_loader: variable: support variable authentication")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>