TFTP booting is slow when a USB keyboard is installed and
stdin has usbkbd added.
This fix is to change Ctrl-C polling for USB keyboard to every second
when NET transfer is running.
My previous patch is expected to be put into usb_kbd_testc(). But it went
into usb_kbd_getc() after applied.
This patch is to put change in correct place.

Signed-off-by: Jim Lin <jilin@nvidia.com>