clk-divider: fix macros

The macro is_power_of_two() in clk-divider.c was defined as !(i & ~i)
which is always true.  Instead use is_power_of_2() from log2.h.

Also add brackets around the macro arguments in div_mask to avoid any
future operator precedence problems.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>
[mturquette@linaro.org: use log2.h per Joe Perches; update changelog]