nvmem: imx-ocotp: Ensure WAIT bits are preserved when setting timing

The i.MX6 and i.MX8 both have a bit-field spanning bits 27:22 called the
WAIT field.

The WAIT field according to the documentation for both parts "specifies
time interval between auto read and write access in one time program. It is
given in number of ipg_clk periods."

This patch ensures that the relevant field is read and written back to the
timing register.

Fixes: 0642bac7 ("nvmem: imx-ocotp: add write support")

Signed-off-by: Bryan O'Donoghue <pure.logic@nexus-software.ie>
Reviewed-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>