efi: libstub: fix up the last remaining open coded boot service call

We use a macro efi_bs_call() to call boot services, which is more
concise, and on x86, it encapsulates the mixed mode handling. This code
does not run in mixed mode, but let's switch to the macro for general
tidiness.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>