USB: musb: fix for crash in DM646x USB when (CPPI)DMA is enabled

Race condition exists between the cppi_interrupt handler and
davinci_interrupt handler w.r.t completing a TX IO.  Since DM646x
has seperate DMA and USB endpoint interrupts cppi_interrupt handler
needs to hold the lock while operating on the endpoint.

Update over previous patch to avoid taking the lock if already
taken.  Tested on DM644x, DM355 and DM646x platforms.

Signed-off-by: Swaminathan S <swami.iyer@ti.com>
Acked-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Acked-by: Anand Gadiyar <gadiyar@ti.com>
Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>