regulator, mc13xxx: Remove pointless test for unsigned less than zero

The variable 'val' is a 'unsigned int', so it can never be less than zero.
This fact makes the "val < 0" part of the test done in BUG_ON() in
mc13xxx_regulator_get_voltage() rather pointles since it can never have
any effect.
This patch removes the pointless test.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Acked-by: Alberto Panizzo <maramaopercheseimorto@gmail.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>