fpga manager: xilinx-spi: remove final dot from dev_err() strings

Most dev_err messages in this file have no final dot. Remove the only two
exceptions to make them consistent.

Reviewed-by: Tom Rix <trix@redhat.com>
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Moritz Fischer <mdf@kernel.org>