usb: gadget: u_serial: Remove extra list operation from gs_start_tx

Commit Ic16cb8ec09c ("usb: gadget: u_serial: Rectify the list operations
is rx/tx path") reworks the list operations to handle the requests
in a proper manner, however gs_start_tx has double list operation which
is incorrect and hence removing it.

Change-Id: I6135cdad5ec5f1922c68e652963d5d8a433a990f
Signed-off-by: Udipto Goswami <quic_ugoswami@quicinc.com>