rpmsg: glink: spi: Update intent before sending cmd

There is a race between updating the intent offset and writing the data
cmd onto the spi bus. If the remote side reads the data cmd and
responds with a rx_done cmd, the intent for that data cmd will be freed
before the offset is updated in glink_spi_send_data.

Move the offset update above the cmd write so all local state updates
are finished before the remote receives any cmds.

Change-Id: I453b7c464a33743fbcb9efee56505b872f2ab804
Signed-off-by: Chris Lew <clew@codeaurora.org>