mtd: rawnand: pasemi: Make pasemi_device_ready() static

The sparse tool complains as follows:

drivers/mtd/nand/raw/pasemi_nand.c:71:5: warning:
 symbol 'pasemi_device_ready' was not declared. Should it be static?

This function is not used outside of pasemi_nand.c, so this commit
marks it static.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200721151657.41027-1-weiyongjun1@huawei.com