ath5k: Fix bitmasks and typos for PCU Diagnostic register

As reported by Ryan Niemi, some bitmasks in the register definition for the PCU
Diagnostic register (DIAG_SW) were missing a zero at the end. While at it fix
some typos and add more comments.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>