xhci: tegra: allocate workqueue for mailbox

xhci: tegra: allocate workqueue for mailbox

Avoid using the shared workqueue as mailbox processing may be delayed
by works scheduled by other drivers.

Allocate a dedicated workqueue for mailbox.

Bug 1691314

DEPENDS ON: < None >

Change-Id: If40bb4263796cb64332dd827cb3bbebcdee5179a
Signed-off-by: TW Chiu <twchiu@nvidia.com>
Reviewed-on: http://git-master/r/839103
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Petlozu Pravareshwar <petlozup@nvidia.com>
GVS: Gerrit_Virtual_Submit
Reviewed-by: Ashutosh Jha <ajha@nvidia.com>