net/sfc/base: move Tx descs number check to generic place

net/sfc/base: move Tx descs number check to generic place

Now we have min/max limits in NIC config, so we can do check against
min/max in a generic place instead of NIC family specific functions.
Check that the descriptors number is a power of 2 is also can be made
common. It removes code duplication and makes NIC family
specific functions a bit shorter.

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