coresight: perform byte counter value check only for non-zero values

A check for the byte counter interrupt value being such that it
divides the memory reserved into equal size blocks is necessary
only when a non-zero value is specified.

Change-Id: I424c09322f2ee82917359d871f0916b3fb50b0be
Signed-off-by: Pushkar Joshi <pushkarj@codeaurora.org>