pinctrl: ti-iodelay: remove redundant pin < 0 check on unsigned int

pin is an unsigned int and therefore can never be < 0 so
this check is redundant. Remove the check and the associated
dev_err error message.

Fixes CoverityScan CID#1396438 ("Unsigned compared against 0")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>