drivers: shs: Fix rps map null checks

Previously SHS did not check rps mask per flush. This adds
checks for a valid RPS map for each flush.

CRs-Fixed: 2349529
Change-Id: Iae1360dd6a9ec2efdbd931576188e3a2c5e69ac5
Acked-by: Raul Martinez <mraul@qti.qualcomm.com>
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>