Fist use extended regexp in order to drop the '\' around the
parentheses which is not supported by BSD sed in regular mode.

Secondly use [[:blank:]] instead of \s, as the later is a GNU
extension.

No functional change intended.

Signed-off-by: Roger Pau Monné <royger@FreeBSD.org>
Reviewed-by: Simon Glass <sjg@chromium.org>