iio: Remove single use of macro definition for regmap name

iio: Remove single use of macro definition for regmap name

There is really no reason for having the `regmap` name as a macro
definition if it is only used once directly in `struct regmap_config`.
It is also more readable this way. Remove these macro definitions and
instead use the string literal directly.

Signed-off-by: Waqar Hameed <waqar.hameed@axis.com>
Link: https://patch.msgid.link/3a8572de8316c7d2746c2ccea8c478f594221319.1748356671.git.waqar.hameed@axis.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>