usb: musb: am35x: fix build warnings

This patch fixes the following build warnings:

drivers/usb/musb/am35x.c:573:12: warning: ‘am35x_suspend’ defined but
not used [-Wunused-function]
drivers/usb/musb/am35x.c:589:12: warning: ‘am35x_resume’ defined but not
used [-Wunused-function]
drivers/usb/musb/am35x.c:573:12: warning: ‘am35x_suspend’ defined but
not used [-Wunused-function]
drivers/usb/musb/am35x.c:589:12: warning: ‘am35x_resume’ defined but not
used [-Wunused-function]

Signed-off-by: Felipe Balbi <balbi@ti.com>