wimax/i2400m: Ensure boot mode cmd and ack buffers are alloc'd before first message

The change to the SDIO boot mode RX chain could try to use the cmd and
ack buffers befor they were allocated.  USB does not have the problem
but both were changed for consistency's sake.

Signed-off-by: Dirk Brandewie <dirk.j.brandewie@intel.com>
Signed-off-by: Inaky Perez-Gonzalez <inaky@linux.intel.com>