staging: wilc1000: remove function pointer wlan_cleanup

This patch removes function pointer wlan_cleanup and just call the function
wilc_wlan_cleanup. Remove static from the function also.
After changing function pointer wlan_cleanup with wilc_wlan_cleanup,
the define wilc_wlan_deinit will be like folowing.

-define wilc_wlan_deinit(nic)  { wilc_wlan_cleanup(); }

The define is unnecessary so just call wilc_wlan_cleanup instead of
wilc_wlan_deinit() and remove the define also.

Signed-off-by: Glen Lee <glen.lee@atmel.com>
Signed-off-by: Tony Cho <tony.cho@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>