cw1200: Fix a signedness bug in cw1200_load_firmware()

The "priv->hw_type" is an enum and in this context GCC will treat it
as an unsigned int so the error handling will never trigger.

Fixes: a910e4a9

 ("cw1200: add driver for the ST-E CW1100 & CW1200 WLAN chipsets")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>