md/raid10: get rid of duplicated conditional expression

Variable 'first' is initialized to zero and updated to @rdev->raid_disk
only if it is greater than 0. Thus condition '>= first' always implies
'>= 0' so the latter is not needed.

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>