iwlwifi: avoid build warning in iwl-core.

When building when CONFIG_IWLWIFI_DEBUG is not set, we get the following
warning:
/work/src/w/drivers/net/wireless/iwlwifi/iwl-core.c: In function ‘iwl_isr’:
/work/src/w/drivers/net/wireless/iwlwifi/iwl-core.c:1707: warning:
unused variable ‘inta_fh’

This patch avoids this warning by adding #ifdef CONFIG_IWLWIFI_DEBUG
before the declaration of inta_fh in iwl_isr() in
drivers/net/wireless/iwlwifi/iwl-core.c

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Acked-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>