net: b44: Remove the unused function __b44_cam_read()

The function __b44_cam_read() is defined in the b44.c file, but not called
elsewhere, so remove this unused function.

drivers/net/ethernet/broadcom/b44.c:199:20: warning: unused function '__b44_cam_read'.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Link: https://bugzilla.openanolis.cn/show_bug.cgi?id=3858
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/20230128090413.79824-1-jiapeng.chong@linux.alibaba.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>