bitmap.h: fix kernel-doc warning and typo

Fix kernel-doc warning in <linux/bitmap.h>:

  include/linux/bitmap.h:341: warning: Function parameter or member 'nbits' not described in 'bitmap_or_equal'

Also fix small typo (bitnaps).

Link: http://lkml.kernel.org/r/0729ea7a-2c0d-b2c5-7dd3-3629ee0803e2@infradead.org
Fixes: b9fa6442

 ("cpumask: Implement cpumask_or_equal()")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>