serial: pxa: Do not tweak clock in pxa serial write() function

The write() function could be used by printk(), which is atomic and
tweaking clock there can cause "BUG: sleeping function called from
invalid context".

Signed-off-by: Bao Haojun <hjbao@marvell.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>