Blackfin Serial Driver: move common variables out of serial headers and into the serial driver

move common variables out of serial headers and into the serial driver and
rename "nr_ports" to "nr_active_ports" so as to easily differentiate
between BFIN_UART_NR_PORTS (the # of available) and nr_ports (the # of enabled)

Signed-off-by: Mike Frysinger <vapier.adi@gmail.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>
Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>