dsp: avtimer: Add adsp ready check before accessing avtimer registers

Add adsp ready check before accessing avtimer registers in
order to avoid NOC errors on these registers during SSR/PDR
usecases while querying offset.

Change-Id: I9246302f01906ab35095c76f729ea082b9756cc5
Signed-off-by: Aditya Bavanari <abavanar@codeaurora.org>