USB NET KL5KUSB101: Fix mem leak in error path of kaweth_download_firmware()

We will leak the storage allocated by request_firmware() if the size of
the firmware is greater than KAWETH_FIRMWARE_BUF_SIZE.
This removes the leak by calling release_firmware() before we return
-ENOSPC.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: David S. Miller <davem@davemloft.net>