rtlwifi: Fix many compile warnings when using W=1

Many warnings like the following arise from a build with W=1 on the
make line:

warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]

Changing the overall debug level storage from unsigned to signed fixes these.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>