net: stmmac: avoid shadowing global buf_sz

stmmac_rx() declares a local variable named "buf_sz" but there is also
a global variable for a module parameter which is called the same. To
avoid confusion, rename the local variable.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Furong Xu <0x1207@gmail.com>
Link: https://patch.msgid.link/E1tpswi-005U6C-Py@rmk-PC.armlinux.org.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>