msm: emac: Cleanup descriptor ring access

Remove usage of any macros to access descriptor words which is
unnecessary. Also, read full 'receive return descriptor' only if
its update flag is set to avoid any race condition.

Change-Id: I42d128836e0d9ea14b26e0433d04436e68ca5f41
Signed-off-by: Niranjana Vishwanathapura <nvishwan@codeaurora.org>