msm_serial_hs: Efficient resource management for RX path

Increase the receive efficiency by providing more DMA resources
and avoiding changing ready for receive line when not necessary.

CRs-Fixed: 648827
Change-Id: Ieebb761cb71373a696a82feb69fdd1e206d1d659
Signed-off-by: Naveen Kaje <nkaje@codeaurora.org>