mtd: spi-nor: Make spi_nor_write_ear() static

commit d75f68a9de742a4b6874f4f6f5840d2a732f8c81 from
https://github.com/Xilinx/linux-xlnx.git

 xlnx_rebase_v6.1

spi_nor_write_ear() is only called from core.c that's why make it static.
And there is of course also missing prototype in header which is reported
by sparse.

Signed-off-by: Michal Simek <michal.simek@amd.com>
State: pending
Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>