efi_loader: refine set_keyboard_layout() status

As per the EFI specification, the HII database protocol function
set_keyboard_layout() must return EFI_INVALID_PARAMETER when it is called
with a NULL key_guid argument. Modify the function accordingly to improve
conformance.

Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Cc: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>