irq: Simplify condition in irq_matrix_reserve()

The if condition in irq_matrix_reserve() can be much simpler.

While at it fix a typo in the comment.

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20210211070953.5914-1-jgross@suse.com