i2400m: remove SDIO device support

SDIO support in this driver was intended to support the iwmc3200
device.  This hardware never became available to normal humans.
Leaving this driver imposes unwelcome maintenance costs for no clear
benefit.

Signed-off-by: John W. Linville <linville@tuxdriver.com>
Acked-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>