slim-msm: Optimize descriptor reading in the rx path

Read and process as many RX DMA descriptors as possible
in a single callback. Handle the case where partial messages
are received and process them later when the complete message
arrives.

Change-Id: I40beb701874d1d2fafc3acf69c144b654ebae74b
Signed-off-by: Naveen Kaje <nkaje@codeaurora.org>