USB: musb: suppress warning about unused flags

Wrap flags with uninitialized_var() to suppress this:

drivers/usb/musb/cppi_dma.c:1158: warning: 'flags' may be used uninitialized
in this function

Signed-off-by: Jon Povey <jon.povey@racelogic.co.uk>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>