USB: musb: fix compilation warning in host only mode

Fixes below compilation warning when host only configuration is
selected.
drivers/usb/musb/musb_core.c: In function 'musb_stage0_irq':
drivers/usb/musb/musb_core.c:711: warning: unused variable 'mbase'

Signed-off-by: Ajay Kumar Gupta <ajay.gupta@ti.com>
Acked-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>