rtc: zynqmp: rework read_offset

rtc: zynqmp: rework read_offset

read_offset() was using static frequency for determining
the tick offset. It was also using remainder from do_div()
operation as tick_mult value which caused the offset to be
incorrect.

At the same time, rework function to improve readability.
It is worth noting, that due to rounding errors, the offset
readback will differ slightly for positive and negative
calibration values.

Reviewed-by: Harini T <harini.t@amd.com>
Tested-by: Harini T <harini.t@amd.com>
Signed-off-by: Tomas Melin <tomas.melin@vaisala.com>
Acked-by: Michal Simek <michal.simek@amd.com>
Link: https://patch.msgid.link/20260122-zynqmp-rtc-updates-v4-3-d4edb966b499@vaisala.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>