staging: wilc1000: remove extern declarations of g_linux_wlan

This patch removes extern declaration of g_linux_wlan from the
followings because it is declared as extern in the wilc_wfi_netdevice.h
file.

- linux_mon.c
- linux_wlan_sdio.c
- wilc_wfi_cfgoperations.c

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