[media] s5p-jpeg: Avoid assigning readl result

Avoid gcc warning when -Wunused-but-set-variable is enabled.
The readl return value need not to be assigned to any variable
as the reading itself is just a part of a sequence required
for clearing the interrupt flag.

Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>