staging: wilc1000: adds a check routine when performed del_key()

This patch is adds a check routine when performed del_key().
We was find this situation while test the 'rmmod sdio'.
That is received inform a .del_key cmd from cfg80211.
If is not exist wep keys, must be ignore a wilc_remove_wep_key() function.
Thus, adds a check routine that key length of wep.

Signed-off-by: Leo Kim <leo.kim@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>