usb:g_dnl:ums: Conditional compilation for mass storage function (f_mass_storage)

The mass storage composite function is now compiled in only when
CONFIG_USB_GADGET_MASS_STORAGE is defined.
Such change provides binary size reduction for boards which use USB
download gadget (like am335x_evm) with DFU, but don't use UMS.

For example at am335x_evm board reduction is more than 2KiB for
text and around 120B for data.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Cc: Marek Vasut <marex@denx.de>