orinoco_usb: Fix broken firmware load error checking

The check of ezusb_firmware_download() return value (added by commit
488ec878) is broken because
ezusb_firmware_download() returns 1 on success.
This causes the driver not to work with the following error:
orinoco_usb: probe of 3-3:1.0 failed with error -14

Check the return value only for negative values.

This fix should be applied to -stable kernels too.

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>