The chunk size used during firmware upload was set to 512, which is
the size of a single SDIO block (or two). This is very inneficient
because we send one or two blocks only per SDIO transaction and don't
get the full benefits of sdio block transfers.
This patch increases the chunk size to 16K. This more than doubles
the transfer speed both in wl127x and wl128x chips, with greater
impact on the latter: