orinoco_usb: clean up some signedness issues

In ezusb_read_ltv() we had a comparison "(bufsize < 0)" which was never
true because bufsize was unsigned.  I looked at the implications of
that.  If we passed a negative number to ezusb_access_ltv() then it
would be used as the size parameter of the memcpy() because that
function uses min_t(int, exp_len, ans_size).

But fortunately when I looked at the callers, bufsize is not controlled
by the user and it's never negative.  So these signedness mistakes have
no impact.

I removed the always false check from ezusb_read_ltv() and I changed the
types in ezusb_access_ltv() and made the variables unsigned.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>