media: sunxi-cir: Remove unnecessary spinlock

Only one register, SUNXI_IR_CIR_REG, is accessed from outside the
interrupt handler, and that register is not accessed from inside it.
As there is no overlap between different contexts, no lock is needed.

Acked-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>