usb: musb: fix PHY power on/off

commi 30a70b02 (usb: musb: fix obex in g_nokia.ko
causing kernel panic) removed phy_power_on()
and phy_power_off() calls from runtime PM callbacks
but it failed to note that the driver depended
on pm_runtime_get_sync() calls to power up the PHY,
thus leaving some platforms without any means to
have a working PHY.

Fix that by enabling the phy during omap2430_musb_init()
and killing it in omap2430_musb_exit().

Fixes: 30a70b02 (usb: musb: fix obex in g_nokia.ko causing kernel panic)
Cc: <stable@vger.kernel.org> # v3.14
Cc: Pali Rohár <pali.rohar@gmail.com>
Cc: Ivaylo Dimitrov <ivo.g.dimitrov.75@gmail.com>
Reported-by: Michael Scott <hashcode0f@gmail.com>
Tested-by: Michael Scott <hashcode0f@gmail.com>
Tested-by: Stefan Roese <sr@denx.de>
Reported-by: Rabin Vincent <rabin@rab.in>
Signed-off-by: Felipe Balbi <balbi@ti.com>