firmware: arm_sdei: fix wrong of_node_put() in init function

After finding a "firmware" dt node arm_sdei tries to match it's
compatible string with it. To do so it's calling of_find_matching_node()
which already takes care of decreasing the refcount on the "firmware"
node. We are then incorrectly decreasing the refcount on that node
again.

This patch removes the unwarranted call to of_node_put().

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>