irq/irq_domain: Quit ignoring error returns from irq_alloc_desc_from().

In commit 4bbdd45a (irq_domain/powerpc: eliminate irq_map; use
irq_alloc_desc() instead) code was added that ignores error returns
from irq_alloc_desc_from() by (silently) casting the return value to
unsigned.  The negitive value error return now suddenly looks like a
valid irq number.

Commits cc79ca69 (irq_domain: Move irq_domain code from powerpc to
kernel/irq) and 1bc04f2c (irq_domain: Add support for base irq and
hwirq in legacy mappings) move this code to its current location in
irqdomain.c

The result of all of this is a null pointer dereference OOPS if one of
the error cases is hit.

The fix: Don't cast away the negativeness of the return value and then
check for errors.

Signed-off-by: David Daney <david.daney@cavium.com>
Acked-by: Rob Herring <rob.herring@calxeda.com>
[grant.likely: dropped addition of new 'irq' variable]
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>