rtlwifi: rtl8192c: Fix W=1 warning

When this driver is built with "make W=1", the following warning occurs:

drivers/net/wireless/rtlwifi/rtl8192c/dm_common.c:907:4: warning: comparison is always false due to limited range of data type [-Wtype-limits]

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