staging: wilc1000: modified NULL check used for 'if' condition in delete_key()

Fix below issue reported by checkpatch.pl script.

'Comparison to NULL could be written "priv->wilc_gtk[key_index]"'
'Comparison to NULL could be written "priv->wilc_ptk[key_index]"'

Signed-off-by: Ajay Singh <ajay.kathat@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>