mtd: nand: denali: remove "Spectra:" prefix from printk strings

As far as I understood from the Kconfig menu deleted by commit
be7f39c5 ("Staging: delete spectra driver"), the "Spectra" is
specific to Intel Moorestown Platform.

The Denali NAND controller IP is used for various SoCs such as
Altera SOCFPGA, Socionext UniPhier, etc.  The platform specific
strings are not preferred in this driver.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>