staging: gdm724x: fix leak at failure path in init_usb()

If an allocation in init_usb() failed, it returns without
deallocation of already allocated resources.

The patch fix it and replaces GFP_ATOMIC by GFP_KERNEL in
alloc_tx_sdu_struct() and alloc_rx_struct() as long as
they are called from probe only.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>