efi_loader: use events for efi_net_receive

A timer event is defined. The timer handler cares for receiving new
packets.

efi_timer_check is called both in efi_net_transmit and efi_net_receive
to enable events during network communication.

Calling efi_timer_check in efi_net_get_status is implemented in a
separate patch.

[agraf] This patch is needed to make efi_net_get_status() actually
        report incoming packets.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
[agraf: fix spelling in comment]
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Alexander Graf <agraf@suse.de>