usb: f_fs: off by one bug in _ffs_func_bind()

usb: f_fs: off by one bug in _ffs_func_bind()

commit 0015f9156092d07b3ec06d37d014328419d5832e upstream.

This loop is supposed to set all the .num[] values to -1 but it's off by
one so it skips the first element and sets one element past the end of
the array.

I've cleaned up the loop a little as well.

Fixes: ddf8abd25994 ('USB: f_fs: the FunctionFS driver')
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
[bwh: Backported to 3.2:
 - Adjust filename, context
 - Add 'i' for iteration but don't bother with 'eps_ptr' as the calculation is
   simpler here]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>