mtd: nand: sunxi: replace the NFC_BUF_TO_USER_DATA() macro by an inline function

sunxi_nfc_user_data_to_buf() is exposed as an inline function, replace the
NFC_BUF_TO_USER_DATA() macro by an inline function to be consistent.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>