sfc: Add parentheses around use of bitfield macro arguments

[ Upstream commit 9724a8504c875145f5a513bb8eca50671cee23b4 ]

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>