sfc_ef100: read Design Parameters at probe time

Several parts of the EF100 architecture are parameterised (to allow
 varying capabilities on FPGAs according to resource constraints), and
 these parameters are exposed to the driver through a TLV-encoded
 region of the BAR.
For the most part we either don't care about these values at all or
 just need to sanity-check them against the driver's assumptions, but
 there are a number of TSO limits which we record so that we will be
 able to check against them in the TX path when handling GSO skbs.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>