wl12xx: increase firmware upload chunk size

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:

wl127x:   512 bytes chunk -> ~132ms
        16384 bytes chunk ->  ~57ms
wl128x:   512 bytes chunk -> ~216ms
        16384 bytes chunk ->  ~37ms

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