net: ipa: introduce gsi_ring_setup()

Prior to IPA v3.5.1, there is no HW_PARAM_2 GSI register, which we
use to determine the number of channels and endpoints per execution
environment.  In that case, we will just assume the number supported
is the maximum supported by the driver.

Introduce gsi_ring_setup() to encapsulate the code that determines
the number of channels and endpoints.

Update GSI_EVT_RING_COUNT_MAX so it is big enough to handle any
available channel for all supported hardware (IPA v4.9 can have 23
channels and 24 event rings).

Signed-off-by: Alex Elder <elder@linaro.org>
Acked-by: AngeloGioacchino Del Regno
Signed-off-by: David S. Miller <davem@davemloft.net>