rtl818x: make dev_alloc_skb() null pointer check to really work

During driver initialization, some skbs are preallocated for RX.
Currenly if the allocation fails, the driver's allocation routine
exits immediatly but it will return zero (success) anyway.

In this way the driver will continue initialization with buggy
pointers around.

This patch makes the driver's allocation routine to return
an error value and to print a complaint message when skb allocation
fails.
In this way its caller will not go further, avoinding the driver to
successfully load, and preventing dereferencing buggy pointers.

An hint is thus printed about why the driver failed.

Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>