Staging: bcm: Fix an integer overflow in IOCTL_BCM_NVM_READ/WRITE

Variables stNVMReadWrite.uioffset and stNVMReadWrite.uiNumBytes
are chosen from userspace and can be very high. The sum of
these two digits would result in a small number. Therefore,
this patch verifies a negative number was not entered, and
reorganizes the equation to remove the integer overflow.

Signed-off-by: Kevin McKinney <klmckinney1@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>