Input: trackpoint - combine calls to ps2_command()

We do not need to call ps2_command() several times in a row, transmitting
every byte as it were a command byte, we can often pack it all in a single
command.

Also, now that ps2_command() handles retransmission, we do not need to do
it ourselves in trackpoint_power_on_reset().

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>