ARM: OMAP: Fix timer posted mode support

ARM: OMAP: Fix timer posted mode support

Currently the dmtimer posted mode is being enabled when the function
omap_dm_timer_enable_posted() is called. This function is only being called
for OMAP1 timers and OMAP2+ timers that are being used as system timers. Hence,
for OMAP2+ timers that are NOT being used as a system timer, posted mode is
not enabled but the "timer->posted" variable is still set (incorrectly) in
the omap_dm_timer_prepare() function.

This is a regression introduced by commit 3392cdd3 (ARM: OMAP: dmtimer:
switch-over to platform device driver) which was before the
omap_dm_timer_enable_posted() function was introduced. Although this is a
regression from the original code it only impacts performance and so is not
needed for stable.

Signed-off-by: Jon Hunter <jon-hunter@ti.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>