net: stmmac: use u8 for ?x_queues_to_use and number_?x_queues

net: stmmac: use u8 for ?x_queues_to_use and number_?x_queues

The maximum number of queues is a compile time constant of only eight.
This makes using a 32-bit quantity wastefulf. Instead, use u8 for
these and their associated variables.

When reading the DT properties, saturdate at U8_MAX. Provided the core
provides DMA capabilities to describe the number of queues, this will
be capped by stmmac_hw_init() with a warning.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Tested-by: Mohd Ayaan Anwar <mohd.anwar@oss.qualcomm.com>
Link: https://patch.msgid.link/E1vzX5K-0000000CVsE-0J0Y@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>