drivers: flash: spi_nor: Print flash size properly when < 1 MiByte

When using SPI-NOR SFDP autodeteection, the flash size printed to
the log is rounded to 1 MiByte. If flash is smaller than 1 MiByte
it is printed as 0 MiByte.

With this patch smaller flash sizes are printed in kiByte, for instance
512 kiByte is printed correctly.

Signed-off-by: Stefan Petersen <spe@ciellt.se>