staging: r8188eu: fix rtw_alloc_hwxmits error detection for now

In _rtw_init_xmit_priv, we use the res variable to store the error
return from the newly converted rtw_alloc_hwxmits function. Sadly, the
calling function interprets res using _SUCCESS and _FAIL still, meaning
we change the semantics of the variable, even in the success case.

This leads to the following on boot:
r8188eu 1-2:1.0: _rtw_init_xmit_priv failed

In the long term, we should reverse these semantics, but for now, this
fixes the driver. Also, inside rtw_alloc_hwxmits remove the if blocks,
as HWXMIT_ENTRY is always 4.

Fixes: f94b47c6

 ("staging: r8188eu: add check for kzalloc")
Signed-off-by: Phillip Potter <phil@philpotter.co.uk>
Link: https://lore.kernel.org/r/20220521204741.921-1-phil@philpotter.co.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>