input: Use finer grain udelays while waitng for the i8042 keyboard buffer to empty

input: Use finer grain udelays while waitng for the i8042 keyboard buffer to empty

On x86, the i8042 keyboard controller driver frequently waits for the keyboard
input buffer to be empty to make sure the controller has had a chance to
process the data it was given. The way the delay loop was structured, if the
controller hadn't cleared the corresponding status bit immediately, it would
wait 1ms before checking again. If the keyboard responded quickly but not
instantly, the driver would still wait a full 1ms when perhaps 1us would have
been sufficient. Because udelay is a busy wait anyway, this change decreases
the delay between checks to 1us.

Also, this change gets rid of a hardcoded 250ms delay.

On Stumpy, this saves 100-150ms during boot.

Signed-off-by: Gabe Black <gabeblack@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@ti.com>