OMAP/serial: Revert bad fix of Rx FIFO threshold granularity

Partially reverts 1776fd05


that introduced regression reported by Paul Walmsley.

This commit restores setting granularity in SCR register
and adds note about comments below being inconsistent with
actual code.

Signed-off-by: Alexey Pelykh <alexey.pelykh@gmail.com>
Tested-by: Paul Walmsley <paul@pwsan.com>
Cc: Kevin Hilman <khilman@linaro.org>
Cc: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>