Staging: rtl8712: signedness bug in init

PollingCnt is 20 and that means we loop 20 times and then run the
timeout code.  After the end of the loop PollingCnt should be -1 but
because it's an unsigned char, it's actually 255 and the timeout
code never runs.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>