sh: intc - primary priority masking fixes

This patch contains various intc fixes for problems reported by
Markus Brunner on the linuxsh-dev mailing list:

http://marc.info/?l=linuxsh-dev&m=118701948224991&w=1



Apart from added comments, the fixes are:

- add intc_set_priority() function prototype to hw_irq.h
- fix off-by-one error in intc_set_priority()
- make sure _INTC_WIDTH() is set for primary priority masking

Big thanks to Markus for finding these problems. Version two fixes
a compile error and an inverted primary check.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Acked-by: Markus Brunner <super.firetwister@gmail.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>