staging: rtlwifi: Fix potential NULL pointer dereference of kzalloc

phydm.internal is allocated using kzalloc which is used multiple
times without a check for NULL pointer. This patch avoids such a
scenario by returning 0, consistent with the failure case.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>