altera_qspi: fix erase and write error code

Fix erase and write error code, which should be "protected".

From the "Embedded Peripherals IP User Guide" of Altera,

The "Illegal write" flag indicates that a write instruction is
targeting a protected sector on the flash memory. This bit is
set to indicate that the IP has cancelled a write instruction.

The "Illegal erase" flag indicates that an erase instruction has
been set to a protected sector on the flash memory. This bit is
set to indicate that the IP has cancelled the erase instruction.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Acked-by: Chin Liang See <clsee@altera.com>
Reviewed-by: Marek Vasut <marex@denx.de>