efi_selftest: Use EFI_SIMPLE_NETWORK_PROTOCOL::GetStatus() for media check

efi_selftest: Use EFI_SIMPLE_NETWORK_PROTOCOL::GetStatus() for media check

According to the UEF specification v2.9, the main purpose of the
EFI_SIMPLE_NETWORK_PROTOCOL::GetStatus() is for checking the link
status via EFI_SIMPLE_NETWORK_MODE::MediaPresent.
So this uses net->get_status() for checking the link status before
running network test.

Signed-off-by: Masami Hiramatsu <masami.hiramatsu@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>