MAINTAINERS: adjust the file entry in SPI OFFLOAD

MAINTAINERS: adjust the file entry in SPI OFFLOAD

Commit 8e02d1886988 ("spi: add basic support for SPI offloading") adds a
new MAINTAINERS section referring to the non-existent file
include/linux/spi/spi-offload.h rather than referring to the files added
with this commit in the directory include/linux/spi/offload/.

Adjust the file reference to the intended directory.

Fixes: 8e02d1886988 ("spi: add basic support for SPI offloading")
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@redhat.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250217092851.17619-1-lukas.bulwahn@redhat.com
Signed-off-by: Mark Brown <broonie@kernel.org>