efi_loader: fix get_package_list_handle() status

When the HII protocol function get_package_list_handle() is called with an
invalid package list handle, it returns EFI_NOT_FOUND but this is not in
its list of possible status codes as per the EFI specification.
Return EFI_INVALID_PARAMETER instead to fix 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: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>