media: atomisp_gmin_platform: stop abusing efivar API

As the code comment already suggests, using the efivar API in this way
is not how it is intended, and so let's switch to the right one, which
is simply to call efi.get_variable() directly after checking whether or
not the GetVariable() runtime service is supported.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>