mtd: spi-nor: update spi_nor_fixups::post_sfdp() documentation

mtd: spi-nor: update spi_nor_fixups::post_sfdp() documentation

After commit 5273cc6df984 ("mtd: spi-nor: core: Call
spi_nor_post_sfdp_fixups() only when SFDP is defined")
spi_nor_post_sfdp_fixups() isn't called anymore if no SFDP is detected.

Update the documentation accordingly.

Fixes: 5273cc6df984 ("mtd: spi-nor: core: Call spi_nor_post_sfdp_fixups() only when SFDP is defined")
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Reviewed-by: Pratyush Yadav <pratyush@kernel.org>
Signed-off-by: Pratyush Yadav (Google) <pratyush@kernel.org>