ASoC: twl6040: Fix comments for register names

Change the register name strings in the comments for the
twl6040_reg table, so it is easier to search for specific
register.

This is cosmetic change.

Before we had for example:
TWL6040_REG_HSLCTL as register definition.

At the register table we had:
TWL6040_HSLCTL

Searching for TWL6040_HSLCTL resulted no hits.

While if we look for REG_HSLCTL, we can find the places
the register has been used.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>