msm: emac: Set default interrupt coalesce value

Update code to use 256us of interrupt coalesce value for both
rx and tx interrupts. Also, retain interrupt coalesce value over
a reset of emac core.

Change-Id: I00a94a8eba663912549f5de64345a67e3df0744d
Signed-off-by: Niranjana Vishwanathapura <nvishwan@codeaurora.org>