xyz-modem: Change getc timeout loop waiting

This fixes the loop delay when using a hw watchdog.

In case a watchdog is used that accesses CPU registers,
the defined delay of 20us in a tight loop will cause a
huge delay in the actual timeout seen. This is caused
by the fact that udelay will inheritantly call WATCHDOG_RESET.
Together with the omap wdt implementation, the seen timeout increases up to
around 30s. This makes the loop very slow and causes long
delays when using the modem.

Instead, implement the 2 sec loop by using the timer interface to know
when to break out of the timeout loop. Watchdog kicking is taken care of
by getc().

Signed-off-by: Tomas Melin <tomas.melin@vaisala.com>