libertas: correct "limited range of data type" warning

  CC [M]  drivers/net/wireless/libertas/wext.o
drivers/net/wireless/libertas/wext.c: In function ‘lbs_get_rts’:
drivers/net/wireless/libertas/wext.c:307: warning: comparison is always
false due to limited range of data type

Signed-off-by: John W. Linville <linville@tuxdriver.com>