mtd: vf610_nfc: specify transfer size before each transfer

Testing showed, that commands like STATUS made the buffer dirty
when executed with NFC_SECSZ set to the page size. It looks
like the controller transfers bogus data when this register
is configured. When setting it to 0, the buffer does not get
altered while the status command still seems to work flawless.

Signed-off-by: Stefan Agner <stefan@agner.ch>