wimax/i2400m: introduce i2400m->bus_setup/release

The SDIO subdriver of the i2400m requires certain steps to be done
before we do any acces to the device, even for doing firmware upload.

This lead to a few ugly hacks, which basically involve doing those
steps in probe() before calling i2400m_setup() and undoing them in
disconnect() after claling i2400m_release(); but then, much of those
steps have to be repeated when resetting the device, suspending, etc
(in upcoming pre/post reset support).

Thus, a new pair of optional, bus-specific calls
i2400m->bus_{setup/release} are introduced. These are used to setup
basic infrastructure needed to load firmware onto the device.

This commit also updates the SDIO subdriver to use said calls.

Signed-off-by: Inaky Perez-Gonzalez <inaky@linux.intel.com>