iwlegacy: use <linux/units.h> helpers

This switches the iwlegacy driver to use celsius_to_kelvin() and
kelvin_to_celsius() in <linux/units.h>.

[akinobu.mita@gmail.com: fix build warnings with format string]
  Link: http://lkml.kernel.org/r/1579014483-9226-1-git-send-email-akinobu.mita@gmail.com
  Link: https://lore.kernel.org/r/20200106171452.201c3b4c@canb.auug.org.au
Link: http://lkml.kernel.org/r/1576386975-7941-10-git-send-email-akinobu.mita@gmail.com
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Kalle Valo <kvalo@codeaurora.org>
Cc: Kalle Valo <kvalo@codeaurora.org>
Cc: Stanislaw Gruszka <sgruszka@redhat.com>
Cc: Amit Kucheria <amit.kucheria@verdurent.com>
Cc: Andy Shevchenko <andy@infradead.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Darren Hart <dvhart@infradead.org>
Cc: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Hartmut Knaack <knaack.h@gmx.de>
Cc: Jean Delvare <jdelvare@suse.com>
Cc: Jens Axboe <axboe@fb.com>
Cc: Johannes Berg <johannes.berg@intel.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: Keith Busch <kbusch@kernel.org>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Luca Coelho <luciano.coelho@intel.com>
Cc: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Cc: Sagi Grimberg <sagi@grimberg.me>
Cc: Sujith Thomas <sujith.thomas@intel.com>
Cc: Zhang Rui <rui.zhang@intel.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>