cassini: Fix a memory leak in the error handling path of cas_init_one()

cas_saturn_firmware_init() allocates some memory using vmalloc(). This
memory is freed in the .remove() function but not it the error handling
path of the probe.

Add the missing vfree() to avoid a memory leak, should an error occur.

Fixes: fcaa4066 ("cassini: use request_firmware")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>