usb: ks_bridge: Fix memory leak in tx data path

ks bridge is allocating urb to transmit data to mdm.
This urb is not getting freed after transmit completes.
Hence call usb_free_urb() after urb gets submitted
successfully. Urb will eventually be freed after tx
completion.

CRs-Fixed: 401536
Change-Id: I7e0bc7d1de74613f6b3b019d1513e22ad9765709
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>