USB: musb: make DBG() calls actually depend on CONFIG_USB_MUSB_DEBUG

Enabling CONFIG_USB_MUSB_DEBUG option causes -DDEBUG to be added to gcc's
command line, however the DBG() macro doesn't depend on DEBUG, so that the
debugging messages get printed regardless of the option, and I don't think
that this was intended. Get rid of otherwise unused xprintk() macro and make
DBG() macro directly call pr_debug() which only results in the actual code
generated if DEBUG is defined.

This change makes musb_hdrc.o ~30% less in size with CONFIG_USB_MUSB_DEBUG
disabled (in host mode).

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>