efi_loader: allow return value in EFI_CALL

Macro EFI_CALL was introduced to call an UEFI function.
Unfortunately it does not support return values.
Most UEFI functions have a return value.

So let's rename EFI_CALL to EFI_CALL_VOID and introduce a
new EFI_CALL macro that supports return values.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>