mx28: Fix typo in POWER_DCLIMITS_NEGLIMIT_OFFSET

The POWER_DCLIMITS_NEGLIMIT_OFFSET bit in mx28 power supply block is
not called POWER_DCLIMITS_NETLIMIT_OFFSET, but POWER_DCLIMITS_NEGLIMIT_OFFSET.
Correct the name in the header file.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>