serial/arc-uart: Handle Rx Error Interrupts w/o any data

Currently, Rx error handling only triggers if there is some Rx data.
Fix that by checking for error - before the data handling.

Reported-by: Mischa Jonker <mjonker@synopsys.com>
Tested-by: Mischa Jonker <mjonker@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>