usb: ehci: tegra: prevent illegal register access

usb: ehci: tegra: prevent illegal register access

Make sure phy is turned ON before reading USB
registers.

Bug 993380
Bug 1006579

Reviewed-on: http://git-master/r/116045
(cherry picked from commit a1a6db7dc88880fb3d4bca0036ce421e4032adae)
Change-Id: If94e691bf9b5b46dd8f8562f27cf86e59a4d6353
Reviewed-on: http://git-master/r/117257
Reviewed-by: Automatic_Commit_Validation_User
GVS: Gerrit_Virtual_Submit
Reviewed-by: Rakesh Bodla <rbodla@nvidia.com>
Tested-by: Rakesh Bodla <rbodla@nvidia.com>
Reviewed-by: Venkat Moganty <vmoganty@nvidia.com>
Rebase-Id: R9c7246e2dae708b1f0f034ccd69c4371848db324