mtd: m25p80: Disable 4KiB erase for s25sl032p, s25sl064p

Quoting from the datasheet for S25FL064P, rev. 05, Nov 18 2011, § 9.17:

"A 64 kB[sic] sector erase (D8h) command issued on 4 kB or 8 kB erase
sectors will erase all sectors in the specified 64 kB region. However,
please note that a 4 kB sector erase (20h) or 8 kB sector erase (40h)
command will not work on a 64 kB sector."

Referring further to Table 8.1 and Table 8.2, it is clearly seen
that most of the sectors are 64KiB; therefore disable this 4KiB
erase support since it's valid only on first/last sectors.

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>