ieee802154: fix new function declaration

The commit 8fad346f

 ("eee802154: add basic support for RF212 to
at86rf230 driver") introduced the new function is_rf212() with some
minor issues in declaration:

1) Fix the function type by changing it to bool as the function
   definition returns a boolean value. Additionally both callers of
   is_rf212() are expected to return a boolean value.

2) Fix the function specifier by deleting the inline keyword as the
   compiler takes care of that.

Signed-off-by: Jean Sacren <sakiwit@gmail.com>
Cc: Phoebe Buckheister <phoebe.buckheister@itwm.fraunhofer.de>
Signed-off-by: David S. Miller <davem@davemloft.net>