serial: omap: fix the reciever line error case

This patch does the following
- In case of errors if there least one data character in the RX FIFO
read it otherwise it may stall the receiver.

This is recommended in the interrupt reset method in the table 23-246 of
the omap4 TRM.

Signed-off-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>
Tested-by: Kevin Hilman <khilman@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>