USB: EHCI: convert warning messages to debug-level

This patch (as1606) converts two warning messages in the ehci-hcd
driver to debug messages, and adds a little extra information to each.

The log messages occur when an EHCI controller takes too long (more
than 20 ms) to turn its async or periodic schedule on or off.  If this
happens at all, it's liable to happen quite often and there's no point
spamming the system log with these warnings.  Furthermore, there's
nothing much we can do about it when the problem happens.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Reported-and-tested-by: Thomas Voegtle <tv@lio96.de>
Cc: stable <stable@vger.kernel.org> # [3.6]
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>