USB: gadget: Enable BAM mode only for BAM2BAM transport

commit ea3e4e39 ("usb: gadget: Enable/Disable USB_BAM based
on composition") enabling USB_BAM irrespective of the transport
used in the driver. Only USB_GADGET_XPORT_BAM2BAM transport was
used for BAM mode. With this change other transports which does
not use BAM also enabling BAM and calling flush_work in function
unbind causing null pointer dereference crash. Hence fix this by
adding transport check for enabling USB_BAM using
bam2bam_func_enabled flag.

Change-Id: Ib2b19e5e9276f2e89d054edda536b590f751cba7
Signed-off-by: Chandana Kishori Chiluveru <cchiluve@codeaurora.org>