[SERIAL] Merge avlab serial board entries in parport_serial

As can be seen from this patch, the avlab_*_[68]50 table entries are
identical to the plain avlab_* entries in every respect.  Hence, there
is no need to list them separately in the pciserial_board nor
parport_pc_pci card tables - they can re-use the plain avlab_* entries.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>