wl12xx: Support routing FW logs to the host

wl12xx: Support routing FW logs to the host

A recently added feature to the firmware enables the driver to retrieve
firmware logs via the host bus (SDIO or SPI).

There are two modes of operation:
1. On-demand: The FW collects its log in an internal ring buffer. This
   buffer can later be read, for example, upon recovery.
2. Continuous: The FW pushes the FW logs as special packets in the RX
   path.

Reading the internal ring buffer does not involve the FW. Thus, as long
as the HW is not in ELP, it should be possible to read the logs, even if
the FW crashes.

A sysfs binary file named "fwlog" was added to support this feature,
letting a monitor process read the FW messages. The log is transferred
from the FW only when available, so the reading process might block.

Signed-off-by: Ido Yariv <ido@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>