misc/bmp085: Enable building as a module

Commit 985087db 'misc: add support for bmp18x chips to the bmp085
driver' changed the BMP085 config symbol to a boolean.  I see no
reason why the shared code cannot be built as a module, so change it
back to tristate.

Fixes: 985087db ("misc: add support for bmp18x chips to the bmp085 driver")
Cc: Eric Andersson <eric.andersson@unixphere.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>