[MTD] fix printk warning

gcc spits out this warning:

drivers/mtd/mtd_blkdevs.c: In function ‘do_blktrans_request’:
drivers/mtd/mtd_blkdevs.c:72: warning: format ‘%ld’ expects type ‘long int’, but argument 2 has type ‘unsigned int’

This could be fixed any number of ways, including use of BUG().
rq_data_dir() only returns 0 or 1, so this entire case is superfluous.
I did the most simple fix.

Signed-off-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>