staging: gdm72xx: simplify alloc_tx_struct and alloc_rx_struct

This patch simplifies alloc_tx_struct and alloc_rx_struct in gdm_sdio.c
and gdm_usb.c by replacing kmalloc+memset with kzalloc and reorganizing
the code.

Signed-off-by: Ben Chan <benchan@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>