pwm: pwm-mediatek: Beautify error messages text

As a cherry-on-top cleanup, make error messages clearer to read
by changing instances of "clock: XXXX failed" to a more readable
"Failed to get XXXX clock". Also add "of" to unsupported period
error.

This is purely a cosmetic change; no "real" functional changes.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Macpaul Lin <macpaul.lin@mediatek.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>