sfc: Convert firmware subtypes to native byte order in efx_mcdi_get_board_cfg()

[ Upstream commit bfeed902946a31692e7a24ed355b6d13ac37d014 ]

On big-endian systems the MTD partition names currently have mangled
subtype numbers and are not recognised by the firmware update tool
(sfupdate).

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
[bwh: Backported to 3.2: use old macros for length of firmware subtype array]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>