iwlwifi: fix compile warning

When iwlwifi is compiled w/o debug, we get
this warning:
iwl-agn.c: In function ‘iwlagn_load_firmware’:
iwl-agn.c:2014: warning: passing argument 3 of ‘iwl_print_hex_dump’ discards qualifiers from pointer target type
iwl-debug.h:73: note: expected ‘void *’ but argument is of type ‘const u8 *’

because the const qualifier is missing in the
inline stub. Fix this.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>