serial: clean up parameter passing for 8250 Rx IRQ handling

The receive_chars() was taking a pointer to a passed in LSR value
in status and knocking off bits as it processed them.  But since
receive_chars isn't returning a value, we can instead pass in
a normal non-pointer value for LSR, and simply return the
residual (unprocessed) LSR once it is done.

The value in this cleanup, is that it clarifies the API of the
receive_chars prior to exporting it to other 8250-like drivers
for shared usage.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>