staging: rtl8188eu: explicitly convert from __le16 to integer

BA_para_set (of type __le16) is implicitly cast into an integer for
bitwise operations, after which the result is converted back into a
__le16. Make the initial cast explicit to make the code clearer. In
addition, refactor the code for readability.

This also removes five sparse warnings of the following type:
drivers/staging/rtl8188eu//core/rtw_mlme_ext.c:6114:70: warning: restricted __le16 degrades to integer

Signed-off-by: Kristina Martšenko <kristina.martsenko@gmail.com>
Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>