mwl8k: mwl8k_update_survey can be static

It fixes following sparse check warning
>#make C=1 CF=-D__CHECK_ENDIAN__ drivers/net/wireless/mwl8k.o
>drivers/net/wireless/mwl8k.c:3089:6: warning: symbol 'mwl8k_update_survey' was not declared. Should it be static?

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Nishant Sarmukadam <nishants@marvell.com>
Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>