staging: rtl8188eu: core: rtw_xmit: Use macros instead of constants

Replace the 0x888e with ETH_P_PAE and 0x0806 with ETH_P_ARP.
These macros can be found in drivers/staging/rtl8192e/rtllib.h

Hexadecimal numbers are not case sensitive,
therefore 0x888e is equal with 0x888E.

The modifications improve the readability of the code.

Signed-off-by: Georgiana Rodica Chelu <georgiana.chelu93@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>