usb: gadget: f_mass_storage: factor out a header file

In order to prepare for the new function interface the f_mass_storage.c
needs to be compiled as a module, and so a header file will be required.

This patch factors out some code to a new f_mass_storage.h.

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>