mtd: spi-nor-core: Fix static checker warnings

Static checker warns 'ret' variable may be used uninitialized in
spi_nor_erase() and spi_nor_write() in case of zero length requests.
Fix these warnings by checking for zero length requests and returning
early.

Reported-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>