ssb: sprom fix some sizes / signedness

Some parts of the sprom struct are bigger than needed.
The leddc and maxpwr values are just 8 bit long and not 16.
rxpo2g and rxpo5g are signed

I got these information for the open source part of the Broadcom SDK
covering sprom version 1 to 9. rxpo2g contained a negative number on my
bcm5354 based device, this cased an error and Broadcom SDK says this is
signed.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>