USB: gadget: fix Blackfin builds after gadget cleansing

The recent change to clean out dead gadget drivers (90f79768

)
missed the call to gadget_is_musbhsfc() behind CONFIG_BLACKFIN.  This
causes Blackfin gadget builds to fail since the function no longer
exists anywhere.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>