Blackfin: add some bounds checking to peripheral_request

The requested peripheral is turned into an index into some state arrays,
so make sure the calculated index doesn't exceed the index.  This occurs
when using bogus pin values or the define headers are screwed up.  Now
we'll notice right away that something needs fixing instead of trying to
track down random memory corruption.

Signed-off-by: Barry Song <barry.song@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>