hwmon: (f75375s) Do not overwrite values read from registers

All bits in the values read from registers to be used for the next
write were getting overwritten, avoid doing so to not mess with the
current configuration.

Signed-off-by: Guillem Jover <guillem@hadrons.org>
Cc: Riku Voipio <riku.voipio@iki.fi>
Cc: stable@kernel.org
Signed-off-by: Jean Delvare <khali@linux-fr.org>