net: ipa: GSI only needs one completion

A mutex ensures we never submit more than one GSI command of any
kind at once.  This means the per-channel and per-event ring
completion structures provide no benefit.  Instead, just use the
single (existing) GSI completion to signal the completion of GSI
commands of all types.

This makes gsi_evt_ring_init() a trivial function with no inverse,
so open-code it in its sole caller and get rid of the function.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>