ieee802154: fix at86rf212_set_txpower() exit path

The commit 9b2777d6 ("ieee802154: add TX power control to
wpan_phy") introduced the new function at86rf212_set_txpower() with
the questionable check of the return of __at86rf230_write() in the
exit path:

1) Both at86rf212_set_txpower() and __at86rf230_write() have the
   same return type.

2) Whatever __at86rf230_write() returns becomes the return value of
   at86rf212_set_txpower().

Thus, fix the exit path by getting rid of that check entirely.

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>