efi_loader: use events for efi_net_receive

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 <[email protected]>
[agraf: fix spelling in comment]
Reviewed-by: Simon Glass <[email protected]>
Signed-off-by: Alexander Graf <[email protected]>