wimax/i2400m: cache firmware on system suspend

In preparation for a reset_resume implementation, have the firmware
image be cached in memory when the system goes to suspend and released
when out.

This is needed in case the device resets during suspend; the driver
can't load firmware until resume is completed or bad deadlocks
happen.

The modus operandi for this was copied from the Orinoco USB driver.

The caching is done with a kobject to avoid race conditions when
releasing it. The fw loader path is altered only to first check for a
cached image before trying to load from disk. A Power Management event
notifier is register to call i2400m_fw_cache() or i2400m_fw_uncache()
which take care of the actual cache management.

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