usb: gadget: rndis: use signed type for a signed value

rndis_get_nr() returns either a non-negative value on success
or a negative value on failure. In case of failure an error code
is returned to the caller of rndis_register().
If the "i" is unsigned, the information about error from rndis_get_nr()
is lost. If there is no error but rndis_get_nr() returns a value greater
than 256 the least significant bits of i are zero effectively limiting the
number of configs to 256.

This patch fixes that.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>