platform/x86: dell-laptop: Fix keyboard max lighting for Dell Latitude E6410

This machine reports number of keyboard backlight led levels, instead of
value of the last led level index. Therefore max_brightness properly needs
to be subtracted by 1 to match led max_brightness API.

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Reported-by: Gabriel M. Elder <gabriel@tekgnowsys.com>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=196913
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>