sfc: Fix Siena mac statistics on big endian platforms

[ Upstream commit a659b2a94d87add999229ecd9f2f56817d5d737b ]

[bwh: Use __force in the one place it's needed]
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>