efi_loader: Prevent dereference of uninitialised variable

efi_loader: Prevent dereference of uninitialised variable

If phandler is returned as NULL from efi_search_protocol then
protocol_interface is never assigned to. Instead return
EFI_UNSUPPORTED as per the spec.

This issue found by Smatch.

Also eliminate the use of the variable protocol_interface as it is not
needed.

Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>