[efi] Disable SNP devices when running iPXE as the application

Some UEFI builds will set up a timer to continuously poll any SNP
devices.  This can drain packets from the network device's receive
queue before iPXE gets a chance to process them.

Use netdev_rx_[un]freeze() to explicitly indicate when we expect our
network devices to be driven via the external SNP API (as we do with
the UNDI API on the standard BIOS build), and disable the SNP API
except when receive queue processing is frozen.

Signed-off-by: Michael Brown <mcb30@ipxe.org>