According to Wikipedia[1]:

  [The indefinite article] "an" is used before words starting with a
  vowel sound, regardless of whether the word begins with a vowel
  letter. This avoids the glottal stop (momentary silent pause) that
  would otherwise be required between a and a following vowel sound.

"UBI" is pronounced like "you-bee-eye" or "youbee", so it starts with a
consonant sound, therefore the correct indefinite article is "a".

[1]: https://en.wikipedia.org/w/index.php?title=English_articles&oldid=928781459#Distinction_between_a_and_an

Signed-off-by: Roland Hieber <rhi@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>