ieee802154/adf7242: fix memory leak of firmware

If the firmware upload or the firmware verification fails then we
printed the error message and exited but we missed releasing the
firmware.

Signed-off-by: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>