firmware: raspberrypi: Fix a leak in 'rpi_firmware_get()'

firmware: raspberrypi: Fix a leak in 'rpi_firmware_get()'

The reference taken by 'of_find_device_by_node()' must be released when
not needed anymore.

Add the corresponding 'put_device()' in the normal and error handling
paths.

Fixes: 4e3d60656a72 ("ARM: bcm2835: Add the Raspberry Pi firmware driver")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/5e17e5409b934cd08bf6f9279c73be5c1cb11cce.1628232242.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>