serial: 8250_pci1xxxx: fix off by one in pci1xxxx_process_read_data()

These > comparisons should be >= to prevent writing one element beyond
the end of the rx_buff[] array.  The rx_buff[] buffer has RX_BUF_SIZE
elements.  Fix the buffer overflow.

Fixes: aba8290f

 ("8250: microchip: pci1xxxx: Add Burst mode reception support in uart driver for writing into FIFO")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/ZZ7vIfj7Jgh-pJn8@moroto
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>