efi_loader: Fix some entry/exit points

When switching between EFI context and U-Boot context we need to swap
the register that "gd" resides in.

Some functions slipped through here, with efi_allocate_pool / efi_free_pool
not doing the switch correctly and efi_return_handle switching too often.

Fix them all up to make sure we always have consistent register state.

Signed-off-by: Alexander Graf <agraf@suse.de>