usb: gadget: f_fs: Accept up to 30 endpoints.

It is allowed by the USB specification to enabled same-address, opposite-
direction endpoints simultaneously, which means 30 non-zero endpoints
are allowed. So double eps_addrmap length to 30.
The original code only accepted 14 descriptors out of a likely intended 15
(as there are 15 endpoint addresses, ignoring direction), because the first
eps_addrmap entry is unused (it is a placeholder for endpoint zero). So
increase eps_addrmap length by one to 31.

Signed-off-by: Vincent Pelletier <plr.vincent@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>