serial: 8250_pci1xxxx: partially revert off by one patch

I was reviewing this code again and I realized I made a mistake here.
It should have been > instead of >=.  The subtract ensures that we
don't go out of bounds.  My patch meant that we don't read the last
chunk of the buffer.

Fixes: 86ee55e9

 ("serial: 8250_pci1xxxx: fix off by one in pci1xxxx_process_read_data()")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/bd6fb361-bbb9-427d-90e8-a5df4de76221@moroto.mountain
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>