driver: gpio: fix shell blink command

The blink command should blink until the user presses a key, however on
some systems a pending keypress is present when entering the loop
causing it to immediate exit before getting a chance to blink.

Add a dummy read to clear the buffer before starting the blink loop.

Signed-off-by: Eric Holmberg <eric.holmberg@northriversystems.co.nz>