w1: mxc_w1: Optimize mxc_w1_ds2_reset_bus()

According to the i.MX reference manual, the reset procedure and
"presence" pulse takes 511 and 512 us, respectively. Measurement for
i.MX27 is about 1100 us. There is no need to wait Reset+Presence
more than this time.
This patch optimizes mxc_w1_ds2_reset_bus() function to use proper
value for delay after w1 bus reset. Nevertheless, a small margin for
the timeout has been added for the case if clock frequency is inaccurate.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>