phy-twl4030-usb: better handle musb_mailbox() failure

setting twl->linkstat = MUSB_UNKNOWN upon error in musb_mailbox as
introduced in
commit 12b7db2b ("usb: musb: Return error value from musb_mailbox")
causes twl4030_usb_irq() to not detect a state change form cable connected
to cable disconnected after such an error so that
pm_runtime_put_autosuspend() will not be called and the usage counter
gets unbalanced. Such errors happen e.g. if the omap2430 module is not
(yet) loaded during plug/unplug events.

This patch introduces a flag instead that indicates whether there is
information for the musb_mailbox pending and calls musb_mailbox() if
that flag is set.

Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>