spl: Refactor spl_load_info->read to use units of bytes

spl: Refactor spl_load_info->read to use units of bytes

Simplify things a bit for callers of spl_load_info->read by refactoring it
to use units of bytes instead of bl_len. This generally simplifies the
logic, as MMC is the only loader which actually works in sectors. It will
also allow further refactoring to remove the special-case handling of
filename.  spl_load_legacy_img already works in units of bytes (oops) so it
doesn't need to be changed.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>