samples: CAN: Tweak GPIO flags in dts for mcp2515

Remove setting GPIO_DIR_OUT in the dts overlay for mcp2515 as a
precursor to new GPIO API work.  The flag isn't used for cs-gpio,
as we hard code GPIO_DIR_OUT in the spi controller code.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>