spi: mpc8xxx: Use get_timer

The comment before the transmission loop in conjunction with the
definition of SPI_TIMEOUT as 1000 implies that the loop is supposed to
have a timeout value of 1000 ms. But since there is no mdelay(1) or
similar in the loop body, the loop just runs 1000 times, without regard
for the time elapsed.

To correct this, use the standard get_timer functionality to properly
time out the loop after 1000 ms.

Signed-off-by: Mario Six <mario.six@gdsys.cc>
Acked-by: Jagan Teki <jagan@amarulasolutions.com>