bcma: keep a direct pointer to the struct device

Accessing struct device is pretty useful/common so having a direct
pointer:
1) Simplifies some code
2) Makes bcma_bus_get_host_dev() unneeded
3) Allows further improvements like using dev_* printing helpers

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>