drivers/sensor: si7006: Mask off low two bits of data reads

The low two bits are not part of the data, but rather "status" bits that
should be masked off.

This is documented in the HTU21D datasheet Edition 8, date 05/2017, pp.
15, and Sensirion SHT21 datasheet version 6, date 10/2022, §6 (wording
exactly the same):

"The two status bits, the last bits of LSB, must be set to ‘0’ before
calculating physical values."

Also Silicon Labs Si7006 example driver code:
        /* Swap the bytes and clear the status bits */
        return ((data.byte[0] * 256) + data.byte[1]) & ~3;

Since these are the LSBs, it has only a small effect and might not have
been noticed despite being wrong.

While editing this code, switch to using the Zephyr endian conversion
functions intead of a written out conversion.

Add error code to error log message.

Signed-off-by: Trent Piepho <tpiepho@gmail.com>