wimax/i2400m: do bootmode buffer management in i2400m_setup/release()

After the introduction of i2400m->bus_setup/release, there is no more
race condition where the bootmode buffers are needed before
i2400m_setup() is called.

Before, the SDIO driver would setup RX before calling i2400m_setup()
and thus need those buffers; now RX setup is done in
i2400m->bus_setup(), which is called by i2400m_setup().

Thus, all the bootmode buffer management can now be done completely
inside i2400m_setup()/i2400m_release(), removing complexity from the
bus-specific drivers.

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