drivers: isdn: get rid of custom strtoul()

There were two methods isdn_gethex() and isdn_getnum() which are custom
implementations of strtoul(). Get rid of them in regard to
strict_strtoul() kernel's function.

Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Hansjoerg Lipp <hjlipp@web.de>
Cc: Tilman Schmidt <tilman@imap.cc>
Cc: Karsten Keil <isdn@linux-pingi.de>
Cc: gigaset307x-common@lists.sourceforge.net
Cc: netdev@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>