OMAP: UART: Fix the revision register read.

The revision register is a 32 bit register. The serial_in() function reads
only the lower 16 bits of the register. This leads to an incorrect computation
of the Module revision.

Signed-off-by: Ruchika Kharwar <ruchika@ti.com>
[oleksandr.savchenko@ti.com: add some whitespaces]
Signed-off-by: Alexander Savchenko <oleksandr.savchenko@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>