usb: musb: fix otg default state

Right after the musb_hdrc driver is loaded, the otg default state
is a_idle, and Mode=Host, which are set by musb_host_setup().

This causes the following kernel message during musb gadget
enumeration.

	CAUTION: musb: Babble Interrupt Occurred

This patch sets the otg default state to b_idle, and its Mode to
Peripheral.

It has been validated on TI AM335x GP EVM USB0 port with g_zero.

Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>