[MTD] CFI: Use 16-bit access to autoselect/read device id data

Recent models of Intel/Sharp and Spansion CFI flash now have significant
bits in the upper byte of device ID codes, read via what Spansion calls
"autoselect" and Intel calls "read device identifier".  Currently these
values are truncated to the low 8 bits in the mtd data structures, as
all CFI read query info has previously been read one byte at a time.
Add a new method for reading 16-bit info, currently just manufacturer
and device codes; datasheets hint at future uses for upper bytes in
other fields.

Signed-off-by: Todd Poynor <tpoynor@mvista.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>