usb: gadget: mbim: Handle SET_ALT from host properly

mbim function driver is setting data_alt_int to the
value 0 received from host upon cable disconnect.
mbim_disable() returns without performing bam data
disconnect if data_alt_int is set to 0. Hence perform
bam data disconnect for SET_ALT 0 in mbim_set_alt().
Also use data_interface_up boolean flag instead of
data_alt_int.

Change-Id: Ie950350b24a5eee83df83989778ee2c5a76e7057
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>