da850-evm, trivial: use da850_evm prefix for consistency

There was a single case of 'da850evm' prefix in the board-da850-evm.c file
where the reset of the prefixes were 'da850_evm'; change it to 'da850_evm' for
consistency.

Signed-off-by: Ben Gardiner <bengardiner@nanometrics.ca>
Acked-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>