IB/iser: Delete an unnecessary variable initialisation in iser_send_data_out()

The variable "tx_desc" will be set to an appropriate pointer a bit later.
Thus omit the explicit initialisation at the beginning.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>