pl011: incorrect assert when fractional divisor is zero

During some refactoring, a2ab9889 introduced a logical bug in the
sanity check of the integer and fractional divisors. This can be seen
with the trivial case of int divisor = 2 and frac divisor = 0 causing
an assert.  Update the check to correctly reflect the comment, and HW
spec/TRM.

Signed-off-by: Luke Starrett <lukes@xsightlabs.com>