mmc: bfin_sdh: fix alloc size for private data

The bfin_sdh driver allocates the wrong size for the private data
in the mmc_host.  The first parameter of mmc_alloc_host should be
the size of the local driver struct rather than the common mmc_host.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Cc: <stable@kernel.org>
Signed-off-by: Chris Ball <cjb@laptop.org>