bus: mhi: ep: Update read pointer only after buffer is written

In the Linux kernel, the following vulnerability has been resolved:

bus: mhi: ep: Update read pointer only after buffer is written

Inside mhi_ep_ring_add_element, the read pointer (rd_offset) is updated before the buffer is written, potentially causing race conditions where the host sees an updated read pointer before the buffer is actually written. Updating rd_offset prematurely can lead to the host accessing an uninitialized or incomplete element, resulting in data corruption.

Invoke the buffer write before updating rd_offset to ensure the element is fully written before signaling its availability.