rtlwifi: rtl8192de: Fix W=1 build warnings

when this driver is built with "make W=1", the following warning is printed:

drivers/net/wireless/rtlwifi/rtl8192de/dm.c:1058:5: 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>