wimax: fix kconfig interactions with rfkill and input layers

WiMAX can work without RFKILL, but it was missing a check to make sure
RFKILL is not being made a module with wimax compiled into the
kernel. This caused failed builds in s390, where CONFIG_INPUT is
always off.

When RFKILL is enabled, the code uses the input layer to report
hardware switch changes; thus, if RFKILL is enabled, INPUT has to be
too. It also needs to display some message when INPUT is disabled that
explains why WiMAX is not selectable.

(issues found by Randy Dunlap in the linux-next tree).

Signed-off-by: Inaky Perez-Gonzalez <inaky@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>