musb: Disable extra prints

There are many musb prints in SPL and U-Boot log.
These prints are required only during musb debug.
So replacing printk with pr_debug in musb_core.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Tested-by: Heiko Schocher <hs@denx.de>
Acked-by: Heiko Schocher <hs@denx.de>