serial: 68328serial.c: remove BAUD_TABLE_SIZE macro

This macro is a duplicate of ARRAY_SIZE defined in kernel api, so just use
that instead.

Signed-off-by: Thiago Farina <tfransosi@gmail.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>