net/sfc/base: fix warnings from VS2015 C compiler (C4245)

net/sfc/base: fix warnings from VS2015 C compiler (C4245)

Fix level 4 warning
"C4245: 'initializing': conversion from 'int' to 'uint32_t',
signed/unsigned mismatch" warning; no functional changes.

Fixes: f9565517ff4f ("net/sfc/base: import filters support")
Fixes: daa007afd04b ("net/sfc/base: split local MAC I/G back into separate flags")
Fixes: 23c6d0dbac80 ("net/sfc/base: improve API to get supported filter matches")
Fixes: 457beb2c4d3e ("net/sfc/base: support filters for encapsulated packets")
Cc: stable@dpdk.org
Signed-off-by: Andrew Lee <alee@solarflare.com>
Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>