musb: fix power field to hold all possible values

MUSB can supply upto 500mA such as, AM3517 and OMAP3EVM Rev >=E and thus
the 'power' field has to hold values above 255.

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