PM / devfreq: bimc-bwmon: Optimize some debug messages

A device register is read and printed using pr_debug. Register reads can be
slow. So, refactor the code such that the register read is also compiled
out if the pr_debug is compiled out.

Change-Id: I4de5ab1f2133e19d6ce0b3cb4547b815526c7abd
Signed-off-by: Saravana Kannan <skannan@codeaurora.org>