usb: musb: host: rely on port_mode to call musb_start()

Currently, we're calling musb_start() twice for DRD ports
in some situations. This has been observed to cause enumeration
issues after suspend/resume cycles with AM335x.

In order to fix the problem, we just have to fix the check
on musb_has_gadget() so that it only returns true if
current mode is Host and ignore the fact that we have or
not a gadget driver loaded.

Fixes: ae44df2e21b5 (usb: musb: call musb_start() only once in OTG mode)
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: <stable@vger.kernel.org> # v3.11+
Tested-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>