fs/ubifs: split long line, use simple asignment

This line has been sitting there unchanged for years now, but it does
not follow current best pratices, that is:
  - do not use imediate assignment,
  - split lines longer than ~80 chars.

Fix that.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>