wimax/i2400m: move i2400m_init() out of i2400m.h

Upcoming changes will have to add things to this function that expose
more internals, which would mean more forward declarators.

Frankly, it doesn't need to be an inline, so moved to driver.c, where
the declarations will be taken from the header file.

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