usb: gadget: storage_common: Make fsg_lun_is_open() a function.

Since function-line macros are to be avoided, this commit replaces
the fsg_lun_is_open() macro with a static inline function.

While at it, this commit also adds “inline” modifier to the
fsg_lun_from_dev() function.

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>