rsi: fix memory leaks and error handling in rsi_91x_usb

The patch fixes a couple of issues:
- absence of deallocation of rsi_dev->rx_usb_urb[0] in the driver;
- potential NULL pointer dereference because of lack of checks for memory
  allocation success in rsi_init_usb_interface().

By the way, it makes rsi_probe() returning error code instead of 1
and fixes comments regarding returning values.

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

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: John W. Linville <linville@tuxdriver.com>