sctp: fix Kconfig bug in default cookie hmac selection

Commit 0d0863b02002 ("sctp: Change defaults on cookie hmac selection")
added a "choice" to the sctp Kconfig file. It introduced a bug which
led to an infinite loop when while running "make oldconfig".

The problem is that the wrong symbol was defined as the default value
for the choice. Using the correct value gets rid of the infinite loop.

Note: if CONFIG_SCTP_COOKIE_HMAC_SHA1=y was present in the input
config file, both that and CONFIG_SCTP_COOKIE_HMAC_MD5=y be present
in the generated config file.

Signed-off-by: Alex Elder <elder@inktank.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>