mtd/progmem: fix incorrect target address calculation

progmem_read/write() is incorrectly calculating the target address,
expecting the offset argument is given in a block number. This is
completely wrong and as a result invalid flash region is accessed.
Byte-oriented read/write interfaces of mtd device accept the
target address in a byte offset, not a block number.

Signed-off-by: Heesub Shin <heesub.shin@samsung.com>