Use obj-m for drivers/net/wireless/ to support driver-select

Use obj-m for drivers/net/wireless/ to support driver-select

The script ./scripts/driver-select allows you to enable only
one wireless driver or a select group of wireless drivers on
drivers/net/wireless/ .

When using driver-select you can opt out to *not* provide a driver
being built in to the directory drivers/net/wireless/ but instead
to a child directory, example would be to enable ATH_COMMON which
only enables modules within drivers/net/wireless/ath/ and nothing
in drivers/net/wireless/ .

When building external kernel modules obj-y enabled directories will
build a built-in.o file only if at least one module was enabled in
that directory. Traditionally the kernel will also build empty
built-in.o files on empty directories. Since running 'make' instead
of 'make modules' makes no sense for external modules we cannot
build a built-in.o on drivers/net/wireless/ by using oby-y and
only enabling modules on children directories, say on
drivers/net/wireless/ath/. Doing this in-kernel (only selecting
ATH_COMMON) *does* work as a first pass 'make' would have built
the empty drivers/net/wireless/built-in.o but for external modules
this is not the case.

To fix this for compat-wireless then use oby-m for drivers/net/wireless/

driver-select now works.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>