serial: uartps: Move the spinlock after the read of the tx empty

Currently we are doing a read of the status register.
Move the spinlock after that as the reads need not be spinlock
protected. This patch prevents relaxing the cpu with spinlock held.

Signed-off-by: Shubhrajyoti Datta <shubhrajyoti.datta@xilinx.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>