Staging: bcm: Check correct user provided length and fix error code returned

bcm driver copies a buffer length provided by userpace without checking it.

RxCntrlMsgBitMask is of type unsigned long so only makes sense to copy
sizeof(unsigned long) bytes.

Also, copy_from_user() returns the number of bytes that could not be copied.
The driver is returning that value as error code instead of -EFAULT.

This patch solves both issues.

Signed-off-by: Javier Martinez Canillas <martinez.javier@gmail.com>
Cc: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>