counter: ti-eqep: Use container_of instead of struct counter_device::priv

counter: ti-eqep: Use container_of instead of struct counter_device::priv

Using counter->priv is a memory read and so more expensive than
container_of which is only an addition. (In this case even a noop
because the offset is 0.)

So container_of is expected to be a tad faster, it's type-safe, and
produces smaller code (ARCH=arm allmodconfig):

	$ source/scripts/bloat-o-meter drivers/counter/ti-eqep.o-pre drivers/counter/ti-eqep.o
	add/remove: 0/0 grow/shrink: 0/9 up/down: 0/-108 (-108)
	Function                                     old     new   delta
	ti_eqep_position_enable_write                132     120     -12
	ti_eqep_position_enable_read                 260     248     -12
	ti_eqep_position_ceiling_write               132     120     -12
	ti_eqep_position_ceiling_read                236     224     -12
	ti_eqep_function_write                       220     208     -12
	ti_eqep_function_read                        372     360     -12
	ti_eqep_count_write                          312     300     -12
	ti_eqep_count_read                           236     224     -12
	ti_eqep_action_read                          664     652     -12
	Total: Before=4598, After=4490, chg -2.35%

Acked-by: David Lechner <david@lechnology.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Link: https://lore.kernel.org/r/4bde7cbd9e43a5909208102094444219d3154466.1640072891.git.vilhelm.gray@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>