mfd: ntxec: Change return type of ntxec_reg8 from __be16 to u16

Register values in NTXEC are big-endian on the I2C bus, but the regmap
subsystem handles the conversion between CPU-endian and big-endian data
internally. ntxec_reg8 should thus return u16, not __be16.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211218152553.744615-1-j.neuschaefer@gmx.net