serial: 8250: Compute ttyS index from port minor

Prepare for 8250 split; calculate the ttyS index directly from
the port minor which avoids the global serial8250_reg reference
from base port operations.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>