spi-mem: fix kernel-doc for spi_mem_dirmap_{read|write}()

The function names in the kernel-doc comments were mistyped, with a word
"dirmap" being repeated twice, so fix them.

Fixes: aa167f3f

 ("spi: spi-mem: Add a new API to support direct mapping")
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Mark Brown <broonie@kernel.org>