usb: dwc2: call dwc2_is_controller_alive() under spinlock

This patch fixes the following problem: data transmission in direction
IN break unless the GSNPSID register access is done with spinlock held.
This issue occurs at least in Exynos4412 SoC, probably in many SoC's
from Exynos familly.

The problem is described here:
https://lkml.org/lkml/2014/12/22/185
And there is linux mailing list discussion:
https://lkml.org/lkml/2015/1/14/17



Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>