wl12xx: use the same SDIO block size for all different chips

The sdio driver uses a block size of 512 bytes by default.  With our
card, this doesn't work correctly because it sets the block size FBR
in the chip too early (ie. before the chip is powered on).  Thus, if
we don't set it explicitly, block mode remains disabled in the chip.
If we try to send more data than fits in one block, the sdio driver
will split it into separate blocks before sending to the chip.  This
causes problems because the chip is not expecting multiple blocks.

At the moment this is not a problem, because we use chunks of 512
bytes for firmware upload and the data is always sent in byte mode.
In the next patch, we will change the chunk size to a bigger value, so
this patch is a preparation for that.

Signed-off-by: Luciano Coelho <coelho@ti.com>