stmmac: remove two useless initialisations

This patch removes two useless initialisations in the
stmmac_rx and stmmac_tx functions.
In the former, the count variable was reset twice and in
the stmmac_tx we only need to increment the dirty pointer
w/o setting the entry variable.

v2: review the subject and comment that was not clear in my
first version.

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>