efi_driver: fix efi_uc_stop()

We must always call EFI_EXIT() when returning from an EFIAPI function.

Fixes: 05ef48a2 ("efi_driver: EFI block driver")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>