i4l: change magic numbers in Eicon DIVA ISDN driver to symbolic names

Replace references to the '20' magic number found throughout the Eicon
ISDN driver for the length of the station_id field in the T30_INFO struct
with the T30_MAX_STATION_ID_LENGTH symbolic constant.

Signed-off-by: Ian Munsie <imunsie@au.ibm.com>
Cc: Armin Schindler <mac@melware.de>
Cc: Karsten Keil <isdn@linux-pingi.de>
Cc: Stoyan Gaydarov <sgayda2@uiuc.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>