staging: gdm72xx: WIMAX_GDM72XX should depend on either USB or MMC

The gdm72xx driver needs to have either the USB or SDIO implementation
enabled to provide useful functionalities, so the driver should depend
on either USB or MMC. This patch makes WIMAX_GDM72XX depend on either
USB or MMC.

Also, WIMAX_GDM72XX needs to be built as a module if its dependent
interface, either USB or MMC, is built as a module. This patch enforces
that in the WIMAX_GDM72XX_USB and WIMAX_GDM72XX_SDIO dependency.

Reported-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Ben Chan <benchan@chromium.org>
Cc: Sage Ahn <syahn@gctsemi.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>