drivers/misc/ad525x_dpot.c: fix typo in spi write16 and write24 transfer counts

This is a bug fix.  Some SPI connected devices using 16/24 bit accesses,
previously failed, now work.

This typo slipped in after testing, during some restructuring.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Cc: Mike Frysinger <vapier@gentoo.org>
Cc: Chris Verges <chrisv@cyberswitching.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>