mwifiex: fix powerpc64-linux- compilation warnings

These warnings can be detected by using powerpc64-linux toolchain
(gcc-4.6.3-nolibc).

  CC [M]  drivers/net/wireless/mwifiex/sta_event.o
drivers/net/wireless/mwifiex/sta_event.c: In function 'mwifiex_process_sta_event':
drivers/net/wireless/mwifiex/sta_event.c:388:4: warning: comparison of distinct pointer types lacks a cast [enabled by default]
  CC [M]  drivers/net/wireless/mwifiex/uap_event.o
drivers/net/wireless/mwifiex/uap_event.c: In function 'mwifiex_process_uap_event':
drivers/net/wireless/mwifiex/uap_event.c:258:11: warning: comparison of distinct pointer types lacks a cast [enabled by default]

Use min_t() instead of min() to fix the warnings.

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