mhi: core: use internal sequence numbers for timesync doorbells

When attempting to latch the remote timer value, host should be
using internal sequence numbers that change every time a client
calls the exposed API and rings the time synchronization
doorbell so that device can identify them as different requests.

Change-Id: I2a38b63c6b942df032f096b17b1fdbc2abdb171b
Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>