char/serial: fix copy&paste error (fifo8_is_full vs empty)

Copy&paste error in serial.c causes a crash when attempting
to read from UART (if there is no data to be read)

Signed-off-by: Vladimir Senkov <hangup@gmail.com>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
