docs: driver-api: fpga: avoid using UTF-8 chars

While UTF-8 characters can be used at the Linux documentation,
the best is to use them only when ASCII doesn't offer a good replacement.
So, replace the occurences of the following UTF-8 characters:

	- U+2014 ('—'): EM DASH

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Moritz Fischer <mdf@kernel.org>