drivers:misc: ti-st: free skb on firmware download

If during validation of the firmware download the data doesn't match what is
expected out of the chip, this calls for a firmware download failure and a
retry.
Free the SKB which collects response during such scenarios.

Signed-off-by: Pavan Savoy <pavan_savoy@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>