usb: musb: cppi41: fix missing unlock on error in cppi41_dma_callback()

Add the missing unlock before return from function cppi41_dma_callback()
in the error handling case.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Felipe Balbi <balbi@ti.com>