mwifiex: fix build error when CONFIG_PM is not set

config: make ARCH=m68k allmodconfig

All error/warnings:

   drivers/net/wireless/mwifiex/cfg80211.c: In function
       'mwifiex_fill_coalesce_rule_info':
>> drivers/net/wireless/mwifiex/cfg80211.c:2493:3: error: implicit
       declaration of function 'mwifiex_is_pattern_supported'
       [-Werror=implicit-function-declaration]
   drivers/net/wireless/mwifiex/cfg80211.c: At top level:
   drivers/net/wireless/mwifiex/cfg80211.c:2537:12: warning:
       'mwifiex_cfg80211_set_coalesce' defined but not used
       [-Wunused-function]
   cc1: some warnings being treated as errors

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>