fpga: Fix of_fpga_mgr_get() reference in fpga_mgr_lock() docs

The fpga_mgr_lock() comment lists of_fpga_mgr_put() as a source of a
manager pointer. The helper which obtains that pointer is
of_fpga_mgr_get(). Correct the reference.

Fixes: ebf877a51ad7 ("fpga: mgr: separate getting/locking FPGA manager")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
Reviewed-by: Xu Yilun <yilun.xu@intel.com>
Link: https://lore.kernel.org/r/20260912100937.86316-1-kmehltretter@gmail.com
Signed-off-by: Xu Yilun <yilun.xu@linux.intel.com>