net/sfc/base: support different event descriptor sizes

net/sfc/base: support different event descriptor sizes

For consistency with the size of Tx descriptors, the size of event
descriptors should be a part of NIC config, not a macro that is
common for all NIC families. Also, add a max number of EvQ buffers
define which is needed to initialize an MCDI buffer at compile time.

Signed-off-by: Igor Romanov <igor.romanov@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>