usb: gadget: f_fs: refactor and document __ffs_ep0_read_events better
usb: gadget: f_fs: refactor and document __ffs_ep0_read_events better

Instead of using variable length array, use a static length equal to
the size of the ffs->ev.types array.  This gets rid of a sparse warning:

	drivers/usb/gadget/function/f_fs.c:401:44: warning:
	Variable length array is used.

and makes it more explicit that the array has a very tight upper size
limit.  Also add some more documentation about the ev.types array and
how its size is limited and affects the rest of the code.

Reported-by: Dan Carpenter <ff341aa343d564f9e53e9dcb6996be8c04859a66@oracle.com>
Reported-by: Rohith Seelaboyina <df8784bf917f56d1149bb39b78a19473b58520b6@nvidia.com>
Signed-off-by: Michal Nazarewicz <605208d9b4c769ca62e9f9f8b592c9cd465fe6d7@mina86.com>
Signed-off-by: Felipe Balbi <94dddeeef08b001e003cce128ddc162a4e2c6cd2@ti.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs