[PATCH] x86_64 irq: Simplfy __assign_irq_vector

By precomputing old_mask I remove an extra if statement, remove an
indentation level and make the code slightly easier to read.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>