cfi_flash: use buffer length in unmap_physmem()

While the flash_detect_legacy() of drivers/mtd/cfi_flash.c
feed unmap_physmem() with MAP_NOCACHE as 2nd parameter,
the do_spi_flash_read_write() of common/cmd_sf.c
feed unmap_physmem() with the length of the mapped buffer
as 2nd parameter.

It's apparently a bug, and I personally think the 2nd parameter
should be the length of the mapped buffer.

Signed-off-by: Kuo-Jung Su <dantesu@faraday-tech.com>
CC: Albert Aribaud <albert.u.boot@aribaud.net>
CC: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>