usb: musb: remove redundant stack buffers

aDate is always the empty string, so entirely pointless. The aRevision
formatting might as well be done as part of the pr_debug() call - that
also avoids it altogether if pr_debug is compiled out.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>