pinctrl: samsung: Use unsigned int for number of controller IO mem resources

Number of IO memory resources cannot be negative obviously and the
driver depends silently on this (by iterating from 0 to
nr_ext_resources+1).  Make this requirement explicit.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>