ti_omap5_common: Respect USB controller number in fastboot

ti_omap5_common: Respect USB controller number in fastboot

On "fastboot reboot-bootloader" we check "dofastboot" variable and do
"fastboot 0" command in U-Boot if it's 1. But there are boards which have
USB controller number other than 0, so it should be respected when
performing "fastboot" command.

This patch reuses CONFIG_FASTBOOT_USB_DEV option toprovide correct USB
controller number to "fastboot" command.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Tom Rini <trini@konsulko.com>