uwb: i1480/GUWA100U: fix firmware download issues

IOGear firmware versions >= 1.4.12224 fail to be downloaded because of a
spurious (and harmless) RCEB received after the download notification. This
patch handles this RCEB and keeps compatibility with future versions that might
not emit this RCEB.

i1480_rceb_check() is reused to check for the RCEB. It is also refactored with
improved comments and reused in another place in mac.c where the checking was
being duplicated.

This patch was tested on both i1480 and GUWA100U HWAs, with all firmware
versions currently available.

Signed-off-by: Anderson Lizardo <anderson.lizardo@indt.org.br>