mtd: Bug in m25p80.c during whole-chip erase

There is a logic error in "whole chip erase" for the m25p80 family.  If
the whole device is successfully erased, erase_chip() will return 0, and
the code will fall through to the "else" clause, and do sector-by-sector
erase in addition to the whole-chip erase.  This patch corrects that.

Also, the MAX_READY_WAIT_COUNT is insufficient for an m25p16 connected
to a 400 MHz powerpc.  Increasing it allows me to successfully program
the device on my board.

Signed-off-by: Steven A. Falco <sfalco@harris.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>