efi_loader: capsule: return a correct error code at find_boot_device()

In case of failure at efi_get_variable_int("BootOrder"), we should
skip examining boot option variables and return an appropriate error
code which is the one the function returned.

Fixes: CID 331153 Code maintainability issues  (UNUSED_VALUE)
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>