'fxas21002c_reg_fields' is only used in '*core*', meaning that '*i2c*'
and '*spi*' complain of a defined but not used const variable.  Let's
move it into the source file.

Fixes the following W=1 kernel build warning(s):

 In file included from drivers/iio/gyro/fxas21002c_i2c.c:14:
 drivers/iio/gyro/fxas21002c.h:79:31: warning: ‘fxas21002c_reg_fields’ defined but not used [-Wunused-const-variable=]
 79 | static const struct reg_field fxas21002c_reg_fields[] = {
 | ^~~~~~~~~~~~~~~~~~~~~
 In file included from drivers/iio/gyro/fxas21002c_spi.c:14:
 drivers/iio/gyro/fxas21002c.h:79:31: warning: ‘fxas21002c_reg_fields’ defined but not used [-Wunused-const-variable=]
 79 | static const struct reg_field fxas21002c_reg_fields[] = {
 | ^~~~~~~~~~~~~~~~~~~~~

Cc: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>