usb: gadget: f_mass_storage: fix mass storage dependency

Legacy gadgets supporting mass storage (g_mass_storage, g_acm_ms, g_multi)
all depend on BLOCK.

Make the standalone compilation of f_mass_storage (without any legacy
gadget) dependent no BLOCK, too.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>