spl_mmc: cleanup variable types

block_read returns unsigned long, so it doesn't make sense to check for
< 0. and neither does marking the header structure as const and then
casting away the constness to load data into it.

Also cleanup some unneeded pointer casting while we're at it.

Signed-off-by: Peter Korsgaard <peter.korsgaard@barco.com>
Reviewed-by: Tom Rini <trini@ti.com>