iwlegacy: mark il_adjust_beacon_interval as noinline

With the new optimized do_div() code, some versions of gcc
produce obviously incorrect code that leads to a link error
in iwlegacy/common.o:

drivers/built-in.o: In function `il_send_rxon_timing':
:(.text+0xa6b4d4): undefined reference to `____ilog2_NaN'
:(.text+0xa6b4f0): undefined reference to `__aeabi_uldivmod'

In a few thousand randconfig builds, I have seen this problem
a couple of times in this file, but never anywhere else in the
kernel, so we can try to work around this in the only file
that shows the behavior, by marking the il_adjust_beacon_interval
function as noinline, which convinces gcc to use the unoptimized
do_div() all the time.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>