net: irda: pxaficp_ir: use sched_clock() for time management

Instead of using directly the OS timer through direct register access,
use the standard sched_clock(), which will end up in OSCR reading
anyway.

This is a first step for direct access register removal and machine
specific code removal from this driver.

This commit changes the behavior, as previously the minimum turnaround
time was counted in 76ns steps, while with this patch it is counted in
microsecond steps. The strictly equal formula would have been :
	    while ((sched_clock() - si->last_clk) * 76 < mtt)

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>