rgb_led_spi: Shorten name/description (drop "mode").

In the 'sdcard_spi' PD/case the SD spec specifically differentiates between
a so-called "SPI mode" and "SD mode" (both being well-defined terms from
the spec), thus the "SPI mode" in the name and description of that PD.

For 'rgb_led_spi' however there's no such distinction for "modes"
really, so shorten the name/description to just 'RGB LED (SPI)'.