EDAC/ti: Fix handling of platform_get_irq() error

platform_get_irq() returns a negative error number on error. In such a
case, comparison to 0 would pass the check therefore check the return
value properly, whether it is negative.

 [ bp: Massage commit message. ]

Fixes: 86a18ee2 ("EDAC, ti: Add support for TI keystone and DRA7xx EDAC")
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Tero Kristo <t-kristo@ti.com>
Link: https://lkml.kernel.org/r/20200827070743.26628-2-krzk@kernel.org