orinoco: Fix walking past the end of the buffer

Fix walking past the end of the bitrate_table array
in the case when the loop counter == BITRATE_TABLE_SIZE.

Reported by: Denis Kirjanov <dkirjanov@kernel.org>
Signed-off-by: David Kilroy <kilroyd@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>