rtl8180: prepare to handle more than two chip types

rtl8180: prepare to handle more than two chip types

Currently a "r8185" integer variable is used as a boolean flag to
indicate whether the card is a rtl8185 or not.
Since now the driver supports only rtl8185 and rtl8180 cards, if
"r8185" variable is zero then the card is implicitly assumed to
be a rtl8180.

Now I'm preparing to add support for a third card type (rtl8187se).

This patch changes the "r8185" flag with an enum variable to
explicitly indicate which card type we have.

I'm submitting this this patch now, even if I still have to submit
other patches that not pertain with rtl8187se support, because
IMHO it's not worth rebasing them on the current code, using r8185
flag, and then changing them back again nearly immediately.
BTW if someone feels I really should do this, please tell me..

Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>