staging: wimax: fix quoted string split across lines in op-rfkill.c

This commit fixes the following checkpatch.pl warning:

WARNING: quoted string split across lines
+		dev_err(dev, "WIMAX_GNL_RFKILL: can't find RFKILL_STATE "
+			"attribute\n");

Signed-off-by: Gabriele Modena <gabriele.modena@gmail.com>
Link: https://lore.kernel.org/r/590fcf33a97025019e5c3b3a915fec9bbe24aa4c.1615652628.git.gabriele.modena@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>