cfi_flash: Fix "Unknown FLASH" error message

When a CFI flash chip could not be detected an error message similar to
the following would be printed on bootup:

 FLASH: ## Unknown FLASH on Bank 1 - Size = 0x01000000 = 0 MB

The printf incorrectly converted the flash size into megabytes.  This
patch fixes the printing of the flash size in megabytes:

 FLASH: ## Unknown FLASH on Bank 1 - Size = 0x01000000 = 16 MB

Signed-off-by: John Schmoller <jschmoller@xes-inc.com>
Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
Signed-off-by: Stefan Roese <sr@denx.de>