Input: i8042 - don't use negation to mark AUX data

Currently i8042_command() negates data coming from the AUX port
of keyboard controller; this is not a very reliable indicator.
Change i8042_command() to fail if response to I8042_CMD_AUX_LOOP
is not coming from AUX channel and get rid of negation.

Based on patch by Vojtech Pavlik.

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>