USB: musb: fix srp sysfs entry deletion

The SRP sysfs attribute is dependent on gadget mode; any
gadget may support SRP.  But "rmmod musb_hdrc" didn't
remove that attribute; fix.

Signed-off-by: Vikram Pandita <vikram.pandita@ti.com>
Acked-by: Felipe Balbi <me@felipebalbi.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>