drivers: sensor: stm32_temp: drop ts-cal-offset property

According to the formulas found in the reference manuals of the SoC
families using the "st,stm32-temp-cal" version of the temperature sensor
(i.e. G0, G4, H7, L0, L1, L4, L5, U5, WB, WL), the temperature is
computed with the following formula:

T = ((TS_CAL2_TEMP - TS_CAL1_TEMP) / (TS_CAL2 - TS_CAL1))
    * (TS_DATA - TS_CAL1) + TS_CAL1_TEMP

What is called ts-cal-offset in the stm32_temp driver is therefore the
same value as TS_CAL1_TEMP1. Use it directly instead of defining another
property.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>