p54: improve eeprom parser

Like other vendors, p54* devices have a checksum for
the EEPROM descriptor data. This patch enhances the
parser code to generate and verify the data fields,
before initializing the radio-chip on the card.

Note:
	If you have to bootstrap an alternative EEPROM image
	for your device and you don't know how to generate a
	valid crc ccitt checksum, you should take a look at:
	http://git.kernel.org/?p=linux/kernel/git/chr/p54tools.git

	The "checksum" utility loads a binary p54 EEPROM blob
	(use the -f switch, to skip the check) and applies
	the correct crc automatically.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>