iwlagn: fix compile warning in iwl5000_gain_computation

The return type of abs() was recently changed from int to long. With
min()'s type checking we thus need to make sure that values of the same
type are compared.

This fixes:

    CC [M]  drivers/net/wireless/iwlwifi/iwl-5000.o
drivers/net/wireless/iwlwifi/iwl-5000.c: In function ‘iwl5000_gain_computation’:
drivers/net/wireless/iwlwifi/iwl-5000.c:320: warning: comparison of distinct pointer types lacks a cast

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Reported-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>