msm: emac: Modify default MTU

Set default MTU to standard 1500 bytes.
Larger MTU size will have more demand for memory
and can lower the performance in some platforms.

Change-Id: I983f3f6b70fbbd3e8dc941b2b15025cf96d250d0
Signed-off-by: Niranjana Vishwanathapura <nvishwan@codeaurora.org>