slim_msm: Reading the smaller length messages properly

If the smaller length messages(<= 4 Bytes) like REPORT_ABSENT
are received over slimbus, the master driver is reading the
excessive length than required. Due to that the next messages
over slimbus are getting corrupted. This patch fixes this issue
and also handles the REPORT_ABSENT message sent by the third party
slimbus client devices.

CRs-Fixed: 431529
Change-Id: I7b3c07a94603482a6a9a7645ade70228144703bc
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>