cnss2: Do MHI force wake for shadow register reading

Unlike shadow register writing, shadow register reading needs MHI in
M0 state. Hence do MHI force wake before reading shadow registers.
Also make sure it is not in atomic context since MHI force wake is
currently not supported it.

Change-Id: I99e9ae1462edc4e805641a5322553b6973074571
Signed-off-by: Yue Ma <yuem@codeaurora.org>