clk: at91: fix clk_programmable_set_parent()

Since commit 1bdf0232 ("clk: at91: make use of syscon/regmap
internally"), clk_programmable_set_parent() is always selecting the
first parent (AKA slow_clk), no matter what's passed in the 'index'
parameter.

Fix that by initializing the pckr variable to the index value.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reported-by: Hans Verkuil <hans.verkuil@cisco.com>
Fixes: 1bdf0232 ("clk: at91: make use of syscon/regmap internally")
Cc: <stable@vger.kernel.org>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Link: lkml.kernel.org/r/1468828152-18389-1-git-send-email-boris.brezillon@free-electrons.com