brcmfmac: Re-enable firmware roaming support

As of 4.18, a firmware that implements the update_connect_params
method but doesn't claim to support roaming causes an error. We
disabled firmware roaming in 4.4 [1] because it appeared to
prevent disconnects, but let's try with the current firmware to see
if things have improved.

[1] https://github.com/raspberrypi/linux/commit/dd9188011786fb62a7960922f31e8e086fb2009b



Signed-off-by: Phil Elwell <phil@raspberrypi.org>