staging: rtl8192u: remove redundant nul check on pointer dev

There is a null check on pointer dev which implies it may be null, however
dev can never be null as it is set in rtl8192_usb_probe via the call
to usb_set_intfdata.

Detected by CoverityScan, CID#143078 ("Dereference after null check")

Fixes: 8fc8598e ("Staging: Added Realtek rtl8192u driver to staging")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>