Some devices don't hook the DVS pin to a GPIO but to ground or VCC.
In those cases, it is not a problem to have no DVS GPIO provided, as the current
code will already switch to software-only DVS selection:

When the DVS GPIO is invalid, lp872x_init_dvs jumps to the set_default_dvs_mode
label, which instructs the chip not to use the DVS pin at all and do it all in
software instead (by clearing the LP8720_EXT_DVS_M bit in the
LP872X_GENERAL_CFG register).

That is reflected later in the code, when setting the bucks (the DVS pin only
applies to the bucks) by checking for the LP8720_EXT_DVS_M bit on the
LP872X_GENERAL_CFG register (in lp872x_select_buck_vout_addr) to decide whether
to use software or hardware DVS selection.

Thus, there is no need to print a warning when the DVS GPIO is invalid.
Signed-off-by: NPaul Kocialkowski <contact@paulk.fr>
Signed-off-by: NMark Brown <broonie@kernel.org>