bcma: signedness bug in bcma_get_next_core()

The u32 would never be less than zero so the error handling would
break.  I changed it to s32 to match how bcma_erom_get_mst_port() is
declared.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>