[MTD] [NOR] cfi_cmdset_0002.c: fix 'cfi_amdstd_erase_varsize' to be static

Make cfi_amdstd_erase_varsize static, as declared at the top
of the file to ensure sparse does not print a warning for an
undeclared function, as so:

drivers/mtd/chips/cfi_cmdset_0002.c:1612:5: warning: symbol 'cfi_amdstd_erase_varsize' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>