efi_loader: Allow bouncing for network

So far bounce buffers were only used for disk I/O, but network I/O
may suffer from the same problem.

On platforms that have problems doing DMA on high addresses, let's
also bounce outgoing network packets. Incoming ones always already
get bounced.

This patch fixes EFI PXE boot on ZynqMP for me.

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