isdn: hisax: Merge Kconfig ifs

The first half of the HiSax config options is presented if
ISDN_DRV_HISAX!=n, while the second half of the options is presented
if ISDN_DRV_HISAX. That's the same, so merge both conditionals.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Karsten Keil <isdn@linux-pingi.de>
Signed-off-by: David S. Miller <davem@davemloft.net>