powerpc/mpc5200: Fix wrong 'no interrupt' handling in of_i2c

If an I2C device node does not specify an interrupt, the .irq member of the
board_info struct was set to -1. This caused crashes on following
irq_dispose_mappings. Leave it NO_IRQ as returned from irq_of_parse_and_map.
(Suggesting -1 as 'i2c-no-irq' used to be a bug in linux/i2c.h.)

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Acked-by: Sean MacLennan <smaclennan@pikatech.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>